Unload plugins in UI config test

[jackwilsdon]

Description

Fixes #4037.

The test_cli_config_file_loads_plugin_commands test uses beets' normal plugin loader to load a test plugin, however it never tidies up after itself. This leads to any future plugin loads being ignored until the next unload_plugins call.

I'm not entirely sure why or how this is passing on our CI at the moment (or on my local machine when running all tests), as it looks like this would have been broken for a while. I'm also not sure why it only affects this test - others should also be impacted by the fact that the zero plugin is not loaded.

This PR changes the config tests so that we always call load_plugins on setup (to store the default beets state) and unload_plugins on teardown (to restore the previously stored state).

You can reproduce the failure by just running these two tests:

$ tox -e py-test test/test_ui.py::ConfigTest::test_cli_config_file_loads_plugin_commands test/test_zero.py::ZeroPluginTest::test_album_art
GLOB sdist-make: /beets/setup.py
py-test inst-nodeps: /beets/.tox/.tmp/package/1/beets-1.5.1.zip
py-test installed: attrs==21.2.0,bcj-cffi==0.5.1,beautifulsoup4==4.9.3,beets @ file:///beets/.tox/.tmp/package/1/beets-1.5.1.zip,Brotli==1.0.9,certifi==2021.5.30,cffi==1.14.6,charset-normalizer==2.0.4,click==8.0.1,confuse==1.5.0,coverage==5.5,Flask==2.0.1,idna==3.2,iniconfig==1.1.1,itsdangerous==2.0.1,jellyfish==0.8.8,Jinja2==3.0.1,MarkupSafe==2.0.1,mediafile==0.7.0,mock==4.0.3,multivolumefile==0.2.3,munkres==1.1.4,musicbrainzngs==0.7.1,mutagen==1.45.1,oauthlib==3.1.1,packaging==21.0,pluggy==1.0.0,py==1.10.0,py7zr==0.16.1,pycparser==2.20,pycryptodomex==3.10.1,pylast==4.2.1,pyparsing==2.4.7,pyppmd==0.16.1,pytest==6.2.5,python-dateutil==2.8.2,python-mpd2==3.0.4,python3-discogs-client==2.3.12,pyxdg==0.27,PyYAML==5.4.1,pyzstd==0.14.4,rarfile==4.0,reflink==0.2.1,requests==2.26.0,requests-oauthlib==1.3.0,responses==0.13.4,six==1.16.0,soupsieve==2.2.1,texttable==1.6.4,toml==0.10.2,Unidecode==1.2.0,urllib3==1.26.6,Werkzeug==2.0.1
py-test run-test-pre: PYTHONHASHSEED='2180953874'
py-test run-test: commands[0] | python -bb -m pytest -rs test/test_ui.py::ConfigTest::test_cli_config_file_loads_plugin_commands test/test_zero.py::ZeroPluginTest::test_album_art
========================================= test session starts =========================================
platform darwin -- Python 3.9.6, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
cachedir: .tox/py-test/.pytest_cache
rootdir: /beets
collected 2 items                                                                                     

test/test_ui.py .                                                                               [ 50%]
test/test_zero.py F                                                                             [100%]

============================================== FAILURES ===============================================
____________________________________ ZeroPluginTest.test_album_art ____________________________________

self = <test.test_zero.ZeroPluginTest testMethod=test_album_art>

    def test_album_art(self):
        self.config['zero']['fields'] = ['images']
    
        path = self.create_mediafile_fixture(images=['jpg'])
        item = Item.from_path(path)
    
        self.load_plugins('zero')
        item.write()
    
        mf = MediaFile(syspath(path))
>       self.assertEqual(0, len(mf.images))
E       AssertionError: 0 != 1

test/test_zero.py:109: AssertionError
===================================== 1 failed, 1 passed in 0.81s =====================================
ERROR: InvocationError for command /beets/.tox/py-test/bin/python -bb -m pytest -rs test/test_ui.py::ConfigTest::test_cli_config_file_loads_plugin_commands test/test_zero.py::ZeroPluginTest::test_album_art (exited with code 1)
_______________________________________________ summary _______________________________________________
ERROR:   py-test: commands failed

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Wow!! As I said in #4037, this is really great detective work—the fix may be simple, but finding it was undoubtedly quite complex. This definitely looks like the right fix. I am also somewhat stumped about this was working in CI and why other tests didn't seem to trigger the same problem, but nonetheless this seems like clearly the right thing to do.

I'll merge this and write a quick changelog entry for packagers.