Centralize test setup (#5362)

This PR deduplicates shared tests setup that was used across test files.
It attempts to centralise the setup into a single source of truth
implementation in `beets.test.helper`.

This works towards the eventual migration to `pytest` (#5361) making it
easier to replace the tests setup with `pytest` fixtures.

It builds upon the temporary files cleanup in #5345.

## Details

### Test setup

* Mostly duplicate test setup implementations in
`beets.test._common.TestCase` and `beets.test.helper.TestHelper` were
merged into a single implementation as
`beets.test.helper.BeetsTestCase`.
* Replaced `unittest.TestCase` and `beets.test.helper.TestHelper`
combination with `beets.test.helper.ImportTestCase`
* Refactored `test/test_plugins.py` removing duplicate import files
setup.
* Centralised setting up the database on disk.
* Centralised plugin test setup into `beets.test.helpers.PluginMixin`.

### Removals

* Around 1/3 of the removed lines correspond to the removal of `def
suite()` functions that were previously used for tests collection.
* Removed redundant `setUp` and `tearDown` methods from many test files
given that the functions that they performed had been centralised in
`beets.test.helper`.
* Removed redundant library initialisations


### Importing

* Centralised import directory creation (only gets created on the first
access).
* Deduplicated `_setup_import_session` implementations in
`TerminalImportSessionSetup` and `ImportHelper`.
* Merged `ImportHelper._create_import_dir` and
`TestHelper.create_importer` implementations into
`ImportHelper.setup_importer`.
* Merged various takes on import files prep into
`ImportHelper.prepare_albums_for_import` and
`ImportHelper.prepare_album_for_import`.
* Seeing that many tests used it, defined `AsIsImporterMixin` which
provides a method `run_asis_importer` to setup _asis_ (non-autotag)
importer and run it

beets/test/_common.py

@@ -15,22 +15,19 @@
 """Some common functionality for beets' test cases."""
 
 import os
-import shutil
 import sys
 import tempfile
-import time
 import unittest
 from contextlib import contextmanager
 
-import beets  # noqa: E402
-import beets.library  # noqa: E402
+import beets
+import beets.library
 
 # Make sure the development versions of the plugins are used
-import beetsplug  # noqa: E402
-from beets import util  # noqa: E402
-from beets import importer, logging  # noqa: E402
-from beets.ui import commands  # noqa: E402
-from beets.util import bytestring_path, syspath  # noqa: E402
+import beetsplug
+from beets import importer, logging, util
+from beets.ui import commands
+from beets.util import syspath
 
 beetsplug.__path__ = [
     os.path.abspath(
@@ -121,9 +118,6 @@ def item(lib=None):
     return i
 
 
-_album_ident = 0
-
-
 def album(lib=None):
     global _item_ident
     _item_ident += 1
@@ -190,100 +184,6 @@ def assert_equal_path(self, a, b):
         )
 
 
-# A test harness for all beets tests.
-# Provides temporary, isolated configuration.
-class TestCase(unittest.TestCase, Assertions):
-    """A unittest.TestCase subclass that saves and restores beets'
-    global configuration. This allows tests to make temporary
-    modifications that will then be automatically removed when the test
-    completes. Also provides some additional assertion methods, a
-    temporary directory, and a DummyIO.
-    """
-
-    def setUp(self):
-        # A "clean" source list including only the defaults.
-        beets.config.sources = []
-        beets.config.read(user=False, defaults=True)
-
-        # Direct paths to a temporary directory. Tests can also use this
-        # temporary directory.
-        self.temp_dir = util.bytestring_path(tempfile.mkdtemp())
-
-        beets.config["statefile"] = os.fsdecode(
-            os.path.join(self.temp_dir, b"state.pickle")
-        )
-        beets.config["library"] = os.fsdecode(
-            os.path.join(self.temp_dir, b"library.db")
-        )
-        beets.config["directory"] = os.fsdecode(
-            os.path.join(self.temp_dir, b"libdir")
-        )
-
-        # Set $HOME, which is used by Confuse to create directories.
-        self._old_home = os.environ.get("HOME")
-        os.environ["HOME"] = os.fsdecode(self.temp_dir)
-
-        # Initialize, but don't install, a DummyIO.
-        self.io = DummyIO()
-
-    def tearDown(self):
-        if os.path.isdir(syspath(self.temp_dir)):
-            shutil.rmtree(syspath(self.temp_dir))
-        if self._old_home is None:
-            del os.environ["HOME"]
-        else:
-            os.environ["HOME"] = self._old_home
-        self.io.restore()
-
-        beets.config.clear()
-        beets.config._materialized = False
-
-
-class LibTestCase(TestCase):
-    """A test case that includes an in-memory library object (`lib`) and
-    an item added to the library (`i`).
-    """
-
-    def setUp(self):
-        super().setUp()
-        self.lib = beets.library.Library(":memory:")
-        self.i = item(self.lib)
-
-    def tearDown(self):
-        self.lib._connection().close()
-        super().tearDown()
-
-
-# Mock timing.
-
-
-class Timecop:
-    """Mocks the timing system (namely time() and sleep()) for testing.
-    Inspired by the Ruby timecop library.
-    """
-
-    def __init__(self):
-        self.now = time.time()
-
-    def time(self):
-        return self.now
-
-    def sleep(self, amount):
-        self.now += amount
-
-    def install(self):
-        self.orig = {
-            "time": time.time,
-            "sleep": time.sleep,
-        }
-        time.time = self.time
-        time.sleep = self.sleep
-
-    def restore(self):
-        time.time = self.orig["time"]
-        time.sleep = self.orig["sleep"]
-
-
 # Mock I/O.
 
 
@@ -388,25 +288,6 @@ def __getattr__(self, key):
         return self.fields.get(key)
 
 
-# Convenience methods for setting up a temporary sandbox directory for tests
-# that need to interact with the filesystem.
-
-
-class TempDirMixin:
-    """Text mixin for creating and deleting a temporary directory."""
-
-    def create_temp_dir(self):
-        """Create a temporary directory and assign it into `self.temp_dir`.
-        Call `remove_temp_dir` later to delete it.
-        """
-        self.temp_dir = bytestring_path(tempfile.mkdtemp())
-
-    def remove_temp_dir(self):
-        """Delete the temporary directory created by `create_temp_dir`."""
-        if os.path.isdir(syspath(self.temp_dir)):
-            shutil.rmtree(syspath(self.temp_dir))
-
-
 # Platform mocking.