rtspserver: Add udp-mcast transport SETUP test
rtspserver: Add udp-mcast transport SETUP test

Refactor utility functions in the test file so they can handle
more than UDP and TCP as lower transport.

https://bugzilla.gnome.org/show_bug.cgi?id=756969

surround-io/gst-rtsp-server,surround-io/gst-rtsp-server,surround-io/gst-rtsp-server