Verify Group Msg reception

[jepenven-silabs]

Problem

While Group Msg reception is supposed to work, it wasn't really tested other than with some Unit Test.

Proposed Solution

Make sure that upon reception of a Group Msg packet (groupId, GroupSession present, etc.) it is routed through the correct path inside the Matter SDK and that no unwanted behaviour is triggered (e.g. sending an Ack).

We can test/achieve this with the TestGroupMessaging yaml test case