Do not require the testing app to be in the apps folder

[phil-davis]

The acceptance tests assume that the testing app is installed in the apps folder.

We want to be more flexible and let it also be installed in a secondary folder that is listed in apps_paths in config.php

Developers (me) might want to do this. e.g. because they want the local apps folder to be "clean" and only contain the apps that are provided in the core repo.

That can be useful, e.g. when locally running code analysis utilities like make test-php-phan - we want it to just analyze code that really is part of the core repo.

[ownclouders]

GitMate.io thinks possibly related issues are #22485 (test), #31579 (test), #22492 (test), #23803 (Can't enable app through category if app folder exists, but app has not been enabled yet), and #4424 (Cannot get multiple app folders/repos to work.).

[phil-davis]

Fixed by PR #34193