MQTT: QoS2 tests and fixes (#4406)

Adds QoS2 related tests and 2 uncovered bug fixes to
#4349.

Still WIP/to come:
 - test that QoS2 session state (pubrel) gets saved/restored. 
- figure out a test and a solution for saving/restoring the PI
mapping/use state when server is restarted

@kozlovic I'd really appreciate suggestions on
- (a) more tests to add (or modify for QoS2), especially WRT cleanup,
clustering
- (b) what to do about `[MQTT-3.10.4-3]`, see comments in
`TestMQTTQoS2InflightMsgsDeliveredAfterUnsubscribe`