test_res_pjsip_session_caps: Create unit test

This unit test runs through combinations of...
	* Local codecs
	* Remote Codecs
	* Codec Preference
	* Incoming/Outgoing

A few new APIs were created to make it easier to test
the functionality but didn't result in any actual
functional change.

ASTERISK_28777

Change-Id: Ic8957c43e7ceeab0e9272af60ea53f056164f164