Use the current v4l2loopback control IDs. Repeat one green frame with GStreamer imagefreeze so the producer continues to supply frames while tests open and close the video device. The existing cleanup stops the producer. Differential Revision: https://phabricator.services.mozilla.com/D325488
See https://developer.mozilla.org/en/docs/Mochitest for detailed information on running and writing mochitests.