test infrastructure deletes entire data dir after test

@bleskes found a sneaky problem that our test infra deletes the entire data dir instead of just it's content. Yet this might not be a problem for 99.9% of our test but the clsuter stats sometimes fail since `File#getTotalSpace()` returns `0` as documented on the javadocs if the path doesn't point to a partition. We should only delete it's content but not the parent.
