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
![img](http://stickerish.com/wp-content/themes/mio/sp-framework/timthumb/timthumb.php?src=http://stickerish.com/wp-content/uploads/2011/03/RageFaceBlackSS.png&h=500&w=500&zc=2&q=100&a=c&s=&f=&cc=&ct=)

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