tests: Cleanup: Use videotestsrc property names instead of numbers

It's self-documenting this way.
stbt.frames: New function to iterate over raw video frames

This commits adds a public API (`stbt.frames`) to access the
`stbt.Display.frames` low-level function used by `detect_match` and
`detect_motion`.

See the self-test `test_using_frames_to_measure_black_screen` for an
example of how you might use this in a real-world test script to measure
the duration of a black screen (e.g. to measure the time it takes to
change channels on a set-top box). You could specify the `mask`
parameter to `cv2.minMaxLoc` if you wanted to mask out UI elements from
what you count as a "black screen". See http://docs.opencv.org for the
documentation on the "cv2" OpenCV python module.