stbt: add a method `process_all_frames` available for the tests

This method forces the pipeline to process all the frames for the duration
of the calls.

This is to use when ad-hoc events are to be captured or when precision is
required.

Conflicts:
	stbt-run
	stbt.py
Completing process_all_frames() to get all the images from a button press to the frame match.
Still to fix/add:
1) Catch up with the live stream.
2) Process the images so as to get a measurement calculated from the images captured during the key press.