[session][test] Extend TestSessionManagerDispatch with group test (#22769)
[session][test] Extend TestSessionManagerDispatch with group test (#22769)

* [session][test] Add group message test vector.

* [session][test] Add short payload secure unicast message test vector.

* [restyle]

* [test] Resolve review comments. Decouple TestInjectGroupSession from nl_unit.

* [test] Fix group name string too large.
project-chip/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip