test_stbt_motiondetect_*: Fix intermittent failures on heavily-loaded systems

On a heavily loaded system, the timeout in `run_motiondetect` was
kicking in before the `stbt-templatematch` GStreamer element had
processed any frames. This problem started to manifest since we run the
unit tests in parallel (see commit 42d1d8aa).

Now we use videotestsrc's `num-buffers` property so that the
`gst-launch` command exits after a certain number of frames have been
processed.
test_wait_for_match_confirm_method_none...: Fix intermittent failure

The glob pattern was catching `screenshot.png` created by another
test being run in parallel; but by the time the `wait_for_match` tried
to *use* that `screenshot.png` file, that other test had finished and
`screenshot.png` had been removed from disk.

This problem started to manifest since commit 42d1d8aa, where we run
the unit tests in parallel.
test_detect_match_times_out_during_yield: Fix intermittent failure

and actually *test* something. Previously the invocation of `stbt-run`
would always succeed (though it was occasionally failing on heavily
loaded systems because `timeout` kicked in before `detect_match` timed
out; now I've removed that timeout because it is completely
unnecessary).

Now the test asserts that `detect_match` only yields a single value,
(the timeout specified in `timeout_secs` should cause the `for` loop to
end after the first iteration).