owr: Add annotations to _owr_schedule_with_hash_table() and pop main context as the thread default
owr: Remove unused request pad removal function

This function's behaviour was also unintuitive and could've
caused too many pads to be released.
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