Remove the @covers fallback from MigrateSourceTestBase::getPluginClass(),...

Dries Buytaertrequested to merge
issue/drupal-3631040:3631040-metadatacollectioniscovers-was-removed into main

Remove the @covers fallback from MigrateSourceTestBase::getPluginClass(), which calls MetadataCollection::isCovers() that PHPUnit 12 removed

Closes #3631040

Merge request reports