minitest: fix rails parallel test runner

[anmarchenko]

What does this PR do?
Fixes rails parallel test executor instrumentation that was broken since I added ci-queue support to minitest framework.

Why was this broken?
In the previous episode of "Datadog figures out how to instrument all possible ways one can use Minitest" we watched minitest-reporters interfering with datadog-ci plugin which caused me to remove plugin and instrument Minitest::CompositeReporter to mark start and end of test sessions.

But... (suspenseful music) there is a twist!
Here is the Minitest.run method (important parts only):

  def self.run args = []
    self.load_plugins unless args.delete("--no-plugins") || ENV["MT_NO_PLUGINS"]

    # ... code omitted for brevity ...

    self.init_plugins options # <------------ here we start session after merging this PR

    self.parallel_executor.start if parallel_executor.respond_to?(:start) # here Rails forks process when using parallel executor
    reporter.start # <---------- here we started session before this PR 
    begin
      __run reporter, options
    rescue Interrupt
      warn "Interrupted. Exiting..."
    end
    self.parallel_executor.shutdown

    reporter.report # <---------- here we end session

    reporter.passed?
  end

Unfortunately as you can see reporter.start is called after the self.parallel_executor.start. This is usually not that important, but not for Rails parallel executor, ladies and gentlemen! When #start is called, it forks processes copying memory and creating an independent process to run your tests.

This worked fine when we used plugin to start session: datadog-ci stores test session in global context that is copied on fork to all child processes. But now, if we start session after workers are forked, then the session is stored only in main test process and not available to workers that actually execute tests.

This PR fixes this issue by moving session start (again) to the init_plugins method but instead of using official Minitest plugin system we are instrumenting this method directly because it is the only way to make it work for all cases discovered so far.

How to test the change?
Manual testing via https://github.com/anmarchenko/openstreetmap-website is required (this is how the issue was discovered). I am happy to pair if anyone would like to see that it works.

This project will be added to Datadog's integration testing environment later.

Why don't you have unit tests for rails parallel executor?
It is extremely tricky to get any data out of forked processes in unit tests, I created a task to investigate it and try figure out a method of creating test environment suitable for unit tests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9acd580) 99.04% compared to head (d255285) 99.04%.

Additional details and impacted files

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##             main     #115   +/-   ##
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  Coverage   99.04%   99.04%           
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  Files         151      152    +1     
  Lines        6676     6691   +15     
  Branches      296      296           
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+ Hits         6612     6627   +15     
  Misses         64       64           

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