Tests: Split the `admin/includesMisc.php` test file to match the new pattern.

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 [434/tests], [57283].

See #53010.

git-svn-id: https://develop.svn.wordpress.org/trunk@62313 602fd350-edb4-49c9-b593-d223f7449a82