Add tests for camera.uvc and fix bugs found in the process

This adds tests for the uvc camera module. It's a good thing too,
because I found a few bugs which are fixed here as well:

 - Graceful handling of non-integer port
 - Failure to take the first host that works when probing host,internalHost
 - Failure to detect if neither of them actually work

This also converts the code to only call add_devices once with a listcomp.