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
What do you mean? At the moment on master trying to run this unit test by itself will fail - there is nothing in bootstrap.php that loads the required apps.