Load apps in tests, fixes #9666

This adds a small file that can be `require`d instead of lib/base.php, which ensures that a minimum set of apps are loaded. This fixes the regression introduced by ac7fb1b23e40e3075535ed5d4188219580b2386a for apps/files_external/tests/mountconfig.php

Fixes #9666
stuff like that shall be moved into the bootstrap.php - I see no need in this change :-1: