chan_pjsip: Add a test event for fast picture updates

This will drive the test on review r3419. Note that the patch for this was
done by Ben Ford, although it was slightly modified for this commit.

ASTERISK-23562
Reported by: Matt Jordan



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@417399 f38db490-d61c-443f-a65b-d21fe96a405b