Peter M. Groen / FlashMQ

When the plugin was enabled, it would never got past the fail of not
having a password file entry. This is fixed.

The reauth didn't work because setting the return data took the path of
first auth, hitting a protection exception. This is fixed.

Writing extended auth tests involved some refactoring, to create a
separate method to parse the auth packet data, because I needed it in
the test too.

Also fix sending an UNSUBACK instead of SUBACK on unsubscribe.

Now that parsing and handling of packets is separated, we can use the
main code to parse packets in the new FlashMQTestClient. This allows
great flexibility in inspecting the server response in a flexible
manner.

We now also have the ability to make tests for MQTT5 features.