ProviderRepositoryTest should execute tests against ProviderRepositoryDecorator

Grepping Drupal core, I'm finding 98 instances of private properties on test classes and 10,198 instances of protected properties. I understand that test classes aren't considered part of Drupal's public API, but if Drupal actually wanted to enforce this, they'd declare test classes as final. I plan to open a core MR requesting the properties on ProviderRepositoryTest be changed to protected.

Automatically closed - issue fixed for 2 weeks with no activity.