fix small bug in testhandler and improve it

* Docs about how to use it
* Have it bind to *all* messages, not just `*.*`
* Deprecate the `get_topic_blah_blah` for a more streamlined API
Since the `get_topic_message_*` methods are destructive, and my tests want to look at all parts of the message, I'm deprecating those in favor of a more direct interface, which is this method + `Message` below.