Fix: the thread continued running after a test failure or it receives a
SIGINT signal (Ctrl+C).

In order to fix this, this commit set the thread as a daemon. The thread
itself calls _mainloop.run(), which blocks it. Only calling
_mainloop.quit() from the main thread is not enough, so a timeout must be
added when there is no signal in the bus within timeout (changed to 0.5
seconds).
Fixed pylint errors from stbt.py
Following the note from
https://developer.gnome.org/glib/2.31/glib-Threads.html#GCond
a loop checking the condition calue has been added because the condition
may not be true even after it returns from `g_cond_wait()`.
This commit makes MessageIterator carry on until the next message by
default, instead of stopping. This simplifies the code and does not impact
on the default "leaky" mode.
Added missing space before "{".
The "sink pipeline" from the unit test
`test_match_consecutive_timed_frames` was specified only because the test
control needs a videotestsrc element in the pipeline. This can be sorted
out by setting `--control=none` instead.
Reduced the size of timer2.mpg video. With such a small screen size and
duration, it shouldn't weight as much as it did.