Add test utilities

Run ./out/<platform>/bin/test-self-view after building to run a simple
audio/video capture and output test.

Run ./out/<platform>/bin/test-send-receive after building to run a test
with one sender and one receiver in the same application.
Then we need to call g_object_ref() here... as whatever you get as parameter in a signal callback is not owned by you. It's owned by the signalling machinery inside GObject.