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.
Considering our discussion about reference ownership, here you would need to pass a new reference to add_remote_candidate(). With the current code that would cause a leak probably.