Added check and cleanup for storage/filecache

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.

Please review @Raydiation @schiesbn @icewind1991 

The warnings should help find out what tests do not clean up properly.
I recently found out that one such test is breaking PHP 5.3 + sqlite tests.
Let's see if they'd pass here.