Fixed autoloader-issue with not-existing-files (regression)

This is a regression after #1190 was fixed and refactored in cad11af82d1856ccf8c6bfd7d4d49efd04ccea81.

Due to the changes in cad11af82d1856ccf8c6bfd7d4d49efd04ccea81, we'll have the same autoloader-issue which led to #1190 when testing folders, e.g.

phpunit Backend/

will result in:
method_exists('Backend/', 'tearDownAfterClass') in vendor/phpunit/phpunit/src/Framework/TestSuite.php:679