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.
And what is also confusing is that we have owr_media_session_set_send_payload() and owr_media_session_add_receive_payload(). add() vs set() for something that sounds like it should be the same.