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
Hehe, those comments were useful, but they were tagged to the wrong line. The line underneath that one included a custom file, tests/base.php, which you (rightly) pointed out should just be in bootstrap.