Some tests don't clean up the file cache and sometimes entries are
reused by mistake in subsequent test suites.
This cleans up the file cache and storage after every test suite and
also shows an annoying warning.
In environments where the data folder is mount from another partition,
it is important to check that the data folder we see is actually the
real one. If the mount failed for some reasons, this fix will make
ownCloud temporarily unavailable instead of causing unpredictable
behavior.
