diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 3e7cd75..64f17f8 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -17,7 +17,7 @@ jobs:
           - '3.0.6'
           - '2.7.8'
           - '2.6.10'
-          # - '2.5.9'  # TODO: Missing `minitest/test_task` in minitest 5.15.0
+          - '2.5.9'  # TODO: Missing `minitest/test_task` in minitest 5.15.0
     steps:
       - uses: actions/checkout@v4
       - name: Set up Ruby
diff --git a/LICENSE-3rdparty.csv b/LICENSE-3rdparty.csv
new file mode 100644
index 0000000..0a3608e
--- /dev/null
+++ b/LICENSE-3rdparty.csv
@@ -0,0 +1,2 @@
+Component,Origin,License,Copyright
+minitest,https://github.com/minitest/minitest,MIT,"Copyright (c) Ryan Davis, seattle.rb"
diff --git a/gemfiles/ruby-2.5.rb b/gemfiles/ruby-2.5.rb
index bf1ab20..f57e674 100644
--- a/gemfiles/ruby-2.5.rb
+++ b/gemfiles/ruby-2.5.rb
@@ -5,6 +5,7 @@
 gem "rake", "~> 13.1.0"
 
 group :test do
+  # Ruby 2.5 is dropped in version 5.16.0
   gem "minitest", "~> 5.15.0"
 end
 
diff --git a/tasks/test.rake b/tasks/test.rake
index b866114..bdfda83 100644
--- a/tasks/test.rake
+++ b/tasks/test.rake
@@ -2,7 +2,12 @@
 
 # @type self: Rake::DSL
 
-require "minitest/test_task"
+begin
+  require "minitest/test_task"
+rescue LoadError
+  # Backport "minitest/test_task" for minitest 5.15.0
+  require_relative "../vendor/minitest/test_task"
+end
 
 Minitest::TestTask.create(:test) do |t|
   t.warning = false
diff --git a/vendor/minitest/test_task.rb b/vendor/minitest/test_task.rb
new file mode 100644
index 0000000..51dde8d
--- /dev/null
+++ b/vendor/minitest/test_task.rb
@@ -0,0 +1,301 @@
+require "shellwords"
+require "rbconfig"
+require "rake/tasklib"
+
+module Minitest # :nodoc:
+
+  ##
+  # Minitest::TestTask is a rake helper that generates several rake
+  # tasks under the main test task's name-space.
+  #
+  #   task <name>      :: the main test task
+  #   task <name>:cmd  :: prints the command to use
+  #   task <name>:deps :: runs each test file by itself to find dependency errors
+  #   task <name>:slow :: runs the tests and reports the slowest 25 tests.
+  #
+  # Examples:
+  #
+  #   Minitest::TestTask.create
+  #
+  # The most basic and default setup.
+  #
+  #   Minitest::TestTask.create :my_tests
+  #
+  # The most basic/default setup, but with a custom name
+  #
+  #   Minitest::TestTask.create :unit do |t|
+  #     t.test_globs = ["test/unit/**/*_test.rb"]
+  #     t.warning = false
+  #   end
+  #
+  # Customize the name and only run unit tests.
+  #
+  # NOTE: To hook this task up to the default, make it a dependency:
+  #
+  #   task default: :unit
+
+  class TestTask < Rake::TaskLib
+    WINDOWS = RbConfig::CONFIG["host_os"] =~ /mswin|mingw/ # :nodoc:
+
+    ##
+    # Create several test-oriented tasks under +name+. Takes an
+    # optional block to customize variables.
+
+    def self.create name = :test, &block
+      task = new name
+      task.instance_eval(&block) if block
+      task.process_env
+      task.define
+      task
+    end
+
+    ##
+    # Extra arguments to pass to the tests. Defaults empty but gets
+    # populated by a number of enviroment variables:
+    #
+    # N (-n flag) :: a string or regexp of tests to run.
+    # X (-e flag) :: a string or regexp of tests to exclude.
+    # A (arg)     :: quick way to inject an arbitrary argument (eg A=--help).
+    #
+    # See #process_env
+
+    attr_accessor :extra_args
+
+    ##
+    # The code to load the framework. Defaults to requiring
+    # minitest/autorun...
+    #
+    # Why do I have this as an option?
+
+    attr_accessor :framework
+
+    ##
+    # Extra library directories to include. Defaults to %w[lib test
+    # .]. Also uses $MT_LIB_EXTRAS allowing you to dynamically
+    # override/inject directories for custom runs.
+
+    attr_accessor :libs
+
+    ##
+    # The name of the task and base name for the other tasks generated.
+
+    attr_accessor :name
+
+    ##
+    # File globs to find test files. Defaults to something sensible to
+    # find test files under the test directory.
+
+    attr_accessor :test_globs
+
+    ##
+    # Turn on ruby warnings (-w flag). Defaults to true.
+
+    attr_accessor :warning
+
+    ##
+    # Optional: Additional ruby to run before the test framework is loaded.
+
+    attr_accessor :test_prelude
+
+    ##
+    # Print out commands as they run. Defaults to Rake's +trace+ (-t
+    # flag) option.
+
+    attr_accessor :verbose
+
+    ##
+    # Use TestTask.create instead.
+
+    def initialize name = :test # :nodoc:
+      self.extra_args   = []
+      self.framework    = %(require "minitest/autorun")
+      self.libs         = %w[lib test .]
+      self.name         = name
+      self.test_globs   = ["test/**/test_*.rb",
+                           "test/**/*_test.rb"]
+      self.test_prelude = nil
+      self.verbose      = Rake.application.options.trace
+      self.warning      = true
+    end
+
+    ##
+    # Extract variables from the environment and convert them to
+    # command line arguments. See #extra_args.
+    #
+    # Environment Variables:
+    #
+    # MT_LIB_EXTRAS :: Extra libs to dynamically override/inject for custom runs.
+    # N             :: Tests to run (string or /regexp/).
+    # X             :: Tests to exclude (string or /regexp/).
+    # A             :: Any extra arguments. Honors shell quoting.
+    #
+    # Deprecated:
+    #
+    # TESTOPTS      :: For argument passing, use +A+.
+    # N             :: For parallel testing, use +MT_CPU+.
+    # FILTER        :: Same as +TESTOPTS+.
+
+    def process_env
+      warn "TESTOPTS is deprecated in Minitest::TestTask. Use A instead" if
+        ENV["TESTOPTS"]
+      warn "FILTER is deprecated in Minitest::TestTask. Use A instead" if
+        ENV["FILTER"]
+      warn "N is deprecated in Minitest::TestTask. Use MT_CPU instead" if
+        ENV["N"] && ENV["N"].to_i > 0
+
+      lib_extras = (ENV["MT_LIB_EXTRAS"] || "").split File::PATH_SEPARATOR
+      self.libs[0,0] = lib_extras
+
+      extra_args << "-n" << ENV["N"]                      if ENV["N"]
+      extra_args << "-e" << ENV["X"]                      if ENV["X"]
+      extra_args.concat Shellwords.split(ENV["TESTOPTS"]) if ENV["TESTOPTS"]
+      extra_args.concat Shellwords.split(ENV["FILTER"])   if ENV["FILTER"]
+      extra_args.concat Shellwords.split(ENV["A"])        if ENV["A"]
+
+      ENV.delete "N" if ENV["N"]
+
+      # TODO? RUBY_DEBUG = ENV["RUBY_DEBUG"]
+      # TODO? ENV["RUBY_FLAGS"]
+
+      extra_args.compact!
+    end
+
+    def define # :nodoc:
+      desc "Run the test suite. Use N, X, A, and TESTOPTS to add flags/args."
+      task name do
+        ruby make_test_cmd, verbose:verbose
+      end
+
+      desc "Print out the test command. Good for profiling and other tools."
+      task "#{name}:cmd" do
+        puts "ruby #{make_test_cmd}"
+      end
+
+      desc "Show which test files fail when run in isolation."
+      task "#{name}:isolated" do
+        tests = Dir[*self.test_globs].uniq
+
+        # 3 seems to be the magic number... (tho not by that much)
+        bad, good, n = {}, [], (ENV.delete("K") || 3).to_i
+        file = ENV.delete("F")
+        times = {}
+
+        tt0 = Time.now
+
+        n.threads_do tests.sort do |path|
+          t0 = Time.now
+          output = `#{Gem.ruby} #{make_test_cmd path} 2>&1`
+          t1 = Time.now - t0
+
+          times[path] = t1
+
+          if $?.success?
+            $stderr.print "."
+            good << path
+          else
+            $stderr.print "x"
+            bad[path] = output
+          end
+        end
+
+        puts "done"
+        puts "Ran in %.2f seconds" % [ Time.now - tt0 ]
+
+        if file then
+          require "json"
+          File.open file, "w" do |io|
+            io.puts JSON.pretty_generate times
+          end
+        end
+
+        unless good.empty?
+          puts
+          puts "# Good tests:"
+          puts
+          good.sort.each do |path|
+            puts "%.2fs: %s" % [times[path], path]
+          end
+        end
+
+        unless bad.empty?
+          puts
+          puts "# Bad tests:"
+          puts
+          bad.keys.sort.each do |path|
+            puts "%.2fs: %s" % [times[path], path]
+          end
+          puts
+          puts "# Bad Test Output:"
+          puts
+          bad.sort.each do |path, output|
+            puts
+            puts "# #{path}:"
+            puts output
+          end
+          exit 1
+        end
+      end
+
+      task "#{name}:deps" => "#{name}:isolated" # now just an alias
+
+      desc "Show bottom 25 tests wrt time."
+      task "#{name}:slow" do
+        sh ["rake #{name} A=-v",
+            "egrep '#test_.* s = .'",
+            "sort -n -k2 -t=",
+            "tail -25"].join " | "
+      end
+    end
+
+    ##
+    # Generate the test command-line.
+
+    def make_test_cmd globs = test_globs
+      tests = []
+      tests.concat Dir[*globs].sort.shuffle # TODO: SEED -> srand first?
+      tests.map! { |f| %(require "#{f}") }
+
+      runner = []
+      runner << test_prelude if test_prelude
+      runner << framework
+      runner.concat tests
+      runner = runner.join "; "
+
+      args  = []
+      args << "-I#{libs.join(File::PATH_SEPARATOR)}" unless libs.empty?
+      args << "-w" if warning
+      args << '-e'
+      args << "'#{runner}'"
+      args << '--'
+      args << extra_args.map(&:shellescape)
+
+      args.join " "
+    end
+  end
+end
+
+class Work < Queue # :nodoc:
+  def initialize jobs = [] # :nodoc:
+    super()
+
+    jobs.each do |job|
+      self << job
+    end
+
+    close
+  end
+end
+
+class Integer # :nodoc:
+  def threads_do(jobs) # :nodoc:
+    q = Work.new jobs
+
+    self.times.map {
+      Thread.new do
+        while job = q.pop # go until quit value
+          yield job
+        end
+      end
+    }.each(&:join)
+  end
+end