Increase the reliability of backing up the mu-plugins directory during tests

[johnbillion]

In order to facilitate switching back to using src as the default directory for testing (#51734) we need to be sure that there's no chance of local files being deleted during a test run.

The tests in tests/phpunit/tests/admin/includesPlugin.php perform a potentially dangerous operation. They back up the mu-plugins directory but then perform assertions before restoring the backup. If an assertion fails, you can lose your mu-plugins directory.

To increase the reliability of these tests:

1. The backup and restore should only happen once, during wpSetUpBeforeClass() and wpTearDownAfterClass()
2. The backup and restore should just rename the directory to save a bunch of file iteration and writing.
3. The assertions should be moved to the end of each test so any test-specific cleanup always runs.

https://core.trac.wordpress.org/ticket/51735