Minor formatting fixes:

stbt-tun: line too long has been cut

stbt.py: Added blank lines to avoid E302 in pylint.
Added one unit test for process_all_frames().

It uses a source with the timestamps overlapped on its frames, sets the
fps to 50, and waits for a specific match with nn-zero milliseconds.
New unit test to check that, after leaving the process_all_frames()
context, the live stream is reset to catch up with the currently displayed
frames.

Because videotestsrc is being used, when it is reset, its timestamps are
reset too. The ideal would be to check that "current_timestamp == 0", but
there is the chance that the first frame is missed. That's the reason for
checking "current_timestamp < match_time".

The function get_live_timestamp() may raise an exception "buffer not
available" if the pipeline has just been reset. To avoid that, a parameter
retry has been added, so that it re-checks until there is a buffer
available.