videotestsrc-time-frame-3.png has been cut to have just the timestamp.

test-stbt-run.sh: fixed identation errors and added failure description
message.
Functional changes:

  1) The way `process_all_frames()` catches up with the live stream has
been changed so that it works with both real streams and `videotestsrc`.
It is also more efficient this way.

Efficiency changes:

  1) The timeouts in the unit tests have been changed so that it takes less
time to run them.

  2) When cathcing up with the live stream, it is more than enough to wait
0.01 seconds while checking the states of the gstreamer elements.
Minor syntax modifications to get the code passing `make check-pylint`.