Refactor TestCustomMessageHandler

- Introduce a new struct for keeping expectations organized.
- Add a boolean field to track whether a response is expected,
  and hence whether a `reply_path` should be included with the response.
- Update Ping and Pong roles for bidirectional communication.
- Introduce panic for when there is no responder and we were expecting
  to include a `reply_path`.
- Refactor `handle_custom_message` code.