Unit test `test_live_stream_caught_up_after_process_all_frames` has been
changed so that it expects a frame from the reset stream.

So as not to get a Gstreamer error during the unit tests running,
reset_source_bin() waits until the sink has been properly reset.

New unit test `test_match_consecutive_timed_frames_in_leaky_mode` has been
added. It tries to wait for 3 consecutives frames without using
stbt.process_all_frames() and checks that this does not succeed.