add camera sensor tests
the following records a 640x480 image on the selfie cam:
v4l2-ctl --device /dev/video0 --set-fmt-video=width=640,height=480,pixelformat=0 v4l2-ctl --device /dev/video0 --stream-mmap --stream-to=test.raw --stream-count=1
dcraw -i test.raw returns 0 when it's a correct raw image and that works (only) for the above. That would be a simple test to avoid regressions, without manually looking into the image data ourselves.