test-sip: C++11'ish and make a better simple call test

* refactor the code in modern C++11
* make a better call test by ensuring that the call is in correct state
  at each step.

Change-Id: I3e17ee59acbf4a5a61783b2bf87eb4914faef65e
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>