[TASK] Clean up and fix unit tests for FAL

Clean up and fix the unit tests that got broken with FAL.
Streamline the API (especially the folder part)
according to the tests.

Change-Id: Idfa54ce69d42621293fda2f4fa079fd20425dd75
Resolves: #36858
Releases: 6.0
Reviewed-on: http://review.typo3.org/11011
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog