Cleanup storage after test, and test .delete_dir().

Cleanup storage after test, and test .delete_dir().

The storage tests work in the system's tmpdir. The python
docs say, we should clean up after using things.  Yes the
directory should be cleaned up on reboot, but if running
tests a lot, the tmpdir could fill up, so we should really
cleanup.

So use the new .delete_dir() on the storage interface to
cleanup test dirs and get them finally removed with
os.rmdir. All nicely packed into cleanup_storage().