local: Free list and all capture sources after notifying the callback about them
transport: Unref rtpbin session after usage
transport: Unset GValues after g_closure_invoke()
transport: Use the generic cclosure marshaller
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.
tests: Update for new API