Changed black frame detection to average-luma based. Lowered threshold.

Changed black frame detection to average-luma based. Lowered threshold. Tested it with a fake webcam and with an external webcam that I covered totally or partially with my finger. Worked as intended.

Also some minor things.
I added numFrames to give an FYI to the user (i.e. to us SWE/SETs debugging troublesome devices) about the final amount of near-black frames out of the total. The test will trigger as failed with as little as one near-black frames, but is interesting to know how many were produced, e.g. just one, or 100%...