[Reviewer: Alex] Add extra tests when forking the call

Alex, can you please review these changes from the AS design updates?

I've also removed the unused app-server module 
I wonder if it would be a good idea to move the request into a new variable here (e.g. `pjsip_msg* voip_req = req; req = NULL;`) to avoid accidentally using it in the code that forks to the native device.