make sure Assignment times out, always
For normal Assignment Request issued by the MSC, there is a call_leg FSM watching over establishing of a call leg.
But less common or future cases may also re-assign while a call is already ongoing.
Also thinkable is an assignment of a non-voice lchan, so that there is no call_leg FSM watching over it.
Furthermore, the call_leg FSM and the mncc_guard timers are in the order of minutes.
Try to fabricate situations in ttcn3 tests where the MSC issues an Assignment Request,
never gets a response, and also fails to take failure action.
If found, add an explicit timeout to the Assignment Request sent to the BSC.