[9.x] Fixes memory leak on PHPUnit's Annotations registry

[nunomaduro]

This pull request addresses one of the memory leaks being mentioned at #44214, by performing a proper cleaning of the PHPUnit's Annotations registry.

On Vapor's test suite, here is the results on 600 tests: 26% less memory used.

// Before
Time: 00:57.879, Memory: 156.50 MB

// After
Time: 00:57.803, Memory: 116.50 MB

The "Before" results, already contains the fixes #44307, #44306, https://github.com/bugsnag/bugsnag-laravel, bugsnag/bugsnag-php#651.