Handle timeout waiting for the device as test failure earlier.

Do not chain screenshare -> webcam services, test one thing at a time.
Use testvideosrc -> v4l2sink gstreamer pipeline as a simple source for
the webcam service.

It better matches the function, since "screenshare" service is not used
in the webcam service anymore.

vivid is an upstream v4l2 device for userspace testing, so it should fit
perfectly for this case. It appears to be more reliable than
v4l2loopback. The only issue is the produced video is dynamic (it has
timestamp rendered on it), so capturing it at source and destination
leads to a slightly different results. I haven't found any option to
disable timestamp, so apply a mask before comparing images.

Additionally, compare I420 images, to avoid (loosy) conversion to RGB
and back.

Those aren't supported here, so ignore them explicitly, instead of
falling back into noisy catch-all statement.

It's blacklisted on Whonix, so skip the test there.