It feels like you have quite a few tests with identical boilerplate, which all mock out getCommandRequest, findFirstAVP and get_diameter_error_code, call a method on self.peer_listener, set up a deferred_errback, and then assert something about the exception raised and the counter.

Could this be extracted out into a common function - so that you'd just call it like test_hss(func=self.peer_listener.fetch_multimedia_auth, first_avp=None, err_code=3004, expected_exception=HSSNotFound, expected_count=0)?