res_pjsip_pubsub: Add test event for state change

This adds a test event when subscription state changes so that
integration tests may trigger new actions at the appropriate times.

Review: https://reviewboard.asterisk.org/r/3383/
........

Merged revisions 411670 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@411671 65c4cc65-6c06-0410-ace0-fbb531ad65f3