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
........

Merged revisions 417399 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@417400 65c4cc65-6c06-0410-ace0-fbb531ad65f3