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.
Don't you think this is confusing if setters on the same object differ in behaviour? It's already super confusing for me with the parse() functions in GstMessage/GstEvent and I always have to look it up in the docs again :)

Maybe we should call it add_send_payload() instead then?