Tests: Rename some newly added files as per the proposed directory structure.

This aims to make the test suite directory and file structure more intuitive for new contributors to work with, as well as follow the PHPUnit recommended test class name pattern with having `Test` as the end of the class name.

Follow-up to [61407], [62096], [62297], [62313], [62328].

See #65208.

git-svn-id: https://develop.svn.wordpress.org/trunk@62342 602fd350-edb4-49c9-b593-d223f7449a82