Always test for newer class names first and fall back to the older.
This improves compatibility when running PHPUnit via a phar while there is also a Composer-installed version of PHPUnit on the system.
Props jrf.
See #52625.
git-svn-id: https://develop.svn.wordpress.org/trunk@50982 602fd350-edb4-49c9-b593-d223f7449a82