Fix 'Drupal.Commenting.ClassComment.Missing' in core/tests

Problem/Motivation

Part of #3105950: Fix 'Drupal.Commenting.ClassComment.Missing' coding standard

Steps toreproduce

Proposed resolution

Comply with Drupal API documentation standards for classes and namespaces and another relevant part of the Drupal coding standards in test modules.
Enable 'Drupal.Commenting.ClassComment.Missing' in core/tests/Drupal/Tests/Core/

Remaining tasks

Review

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet