Refactor test_plugin.py: Streamline plugin tests and migrate to pytest

[semohr]

This PR addresses historical testing debt in test_plugins.py by standardizing plugin test infrastructure.

• Introduces PluginTestCasePytest, deprecating the older PluginTestCase (unittest-based) to unify tests under pytest.
• Enhances PluginMixin with programmatic plugin registration/unregistration while still working through standard load_plugins, improving test isolation and avoiding fragile disk/module hacks.
• Simplifies and modernizes individual tests where possible, reducing boilerplate and making future plugin tests easier to extend.

There is still the PromptChoicesTest left which I have not touched (this one scares me 👻).

Coverage:

# before
Name               Stmts   Miss Branch BrPart   Cover   Missing
---------------------------------------------------------------
beets/plugins.py     228     57     70      6  70.13%   119, 131-136, 184->186, 186->188, 188->191, 279-282, 299-305, 315-321, 352, 366-387, 473-475, 489, 523-529, 544-547, 575-578, 595-607
---------------------------------------------------------------
TOTAL                228     57     70      6  70.13%

# after
Name               Stmts   Miss Branch BrPart   Cover   Missing
---------------------------------------------------------------
beets/plugins.py     228     45     70      8  74.83%   131-136, 184->186, 186->188, 188->191, 279-282, 286->exit, 299-305, 315-321, 352, 372->387, 473-475, 489, 523-529, 544-547, 575-578, 595-607
---------------------------------------------------------------
TOTAL                228     45     70      8  74.83%

[sourcery-ai]

Reviewer's Guide

This PR refactors the plugin test suite by migrating from unittest to pytest (introducing PluginTestCasePytest and deprecating the old TestCase), augments the PluginMixin helper with dynamic registration/unregistration and context managers for better isolation, and modernizes individual tests through fixtures, parametrization, and reduced boilerplate.

File-Level Changes

Change	Details	Files
Introduce pytest-based test harness and deprecate unittest infrastructure	Add PluginTestCasePytest with an autouse fixture for setup/teardown Switch test classes to inherit from PluginTestCasePytest instead of PluginTestCase Remove setUp methods and migrate to pytest fixtures and markers	test/test_plugins.py beets/test/helper.py
Enhance PluginMixin for dynamic plugin management	Implement register_plugin/unregister_plugin/unregister_all_plugins to manage sys.modules entries Add plugins() context manager for scoped plugin loading and unloading Provide get_plugin_instance helper and improve load_plugins/unload_plugins behavior	beets/test/helper.py
Modernize individual plugin tests with fixtures, context managers, and parametrization	Wrap plugin-specific tests in self.plugins(...) context manager instead of manual registration Replace repetitive listener tests with @pytest.mark.parametrize and simplified assertions Simplify prompt choice and import-plugin tests to use with self.plugins and pytest fixtures	test/test_plugins.py

Possibly linked issues

• Error when running poetry run pytest #5704: The PR refactors plugin tests to use plugins.load_plugins and migrates to pytest, directly addressing the issue of using regular plugin loading in tests.

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[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.48%. Comparing base (394d78d) to head (8de2a14).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6022      +/-   ##
==========================================
- Coverage   66.52%   66.48%   -0.04%     
==========================================
  Files         117      117              
  Lines       18084    18084              
  Branches     3061     3061              
==========================================
- Hits        12030    12023       -7     
- Misses       5403     5409       +6     
- Partials      651      652       +1     

see 1 file with indirect coverage changes

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