_serve_once treated a cancelled call future as finished, so an event loop
cancelling all tasks at once (shutdown) never sent the worker a cancel and the
call's thread kept asyncio.run waiting for SILENCE_S. The cameras audio test
now removes its voice flow and device, the unpaired-frame test waits for its
first frame before the unpair, and test_connector no longer inherits the
controller an app test bound to demand.