res_rtp_asterisk: Raise event when RTP port is allocated

This change raises a testsuite event to provide what port
Asterisk has actually allocated for RTP. This ensures that
testsuite tests can remove any assumption of ports and instead
use the actual port in use.

ASTERISK-28070

Change-Id: I91bd45782e84284e01c89acf4b2da352e14ae044