test broker: Subscription Identifier handling

[jonesmz]

https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901120

It is a Protocol Error for a PUBLISH packet to contain any Subscription Identifier other than those received in SUBSCRIBE packet which caused it to flow. A PUBLISH packet sent from a Client to a Server MUST NOT contain a Subscription Identifier [MQTT-3.3.4-6].

The test broker should disconnect client's that send a PUBLISH packet with a Subscription Identifier.

The client should assert that the Subscription Identifier received in a publish packet matches a subscription identifier that the client established with the server.

[kleunen]

This issue still seems relevant for the broker as well, right ?

Maybe disconnect is a bit brutal, maybe generate a warning when client Publish contains Subscription Identifier and strip the Subscription Identifier ?

And maybe it is a bit too much overhead for the client to check every incoming publish ?

[redboltz]

I just create #762. Some of invalid combination of flags and properties can be checked without extra checking loop.
I didn't design it yes but I guess it works well.