[Symfony 4] EntityResourceValidationTraitTest should not get a mock for an internal trait.

Problem/Motivation

EntityResourceValidationTraitTest calls

    $trait = $this->getMockForTrait('Drupal\rest\Plugin\rest\resource\EntityResourceValidationTrait');

EntityResourceValidationTrait is marked @internal. In Symfony 4, the class loader looks at usages of @internal classes and ensures that the class and the class user are in the same root namespace (i.e. \Drupal). Because the mock exists outside of the \Drupal namespace, this triggers a deprecation error on Symfony 4. Setting to major as we should fix this prior to branching 9.0.x

Proposed resolution

If this issue were prevalent, I would consider trying to fix it in the Symfony class loader, or in our own deprecation listener or in some other global fashion. The actual deprecation error is triggered during a class scan after the test is over, so we can't wrap the ->getMockForTrait method in a custom error handler. Since this one test is the only place where this is an issue, the easiest solution is to replace the mock with a wrapper test class for the trait.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet