test_wait_for_match_no_match: Don't test for image path search behaviour

That is covered by the tests
test_wait_for_match_searches_in_script_directory,
test_press_until_match_searches_in_script_directory,
test_detect_match_searches_in_script_directory,
test_detect_match_searches_in_library_directory, and
test_detect_match_searches_in_caller_directory.

There is no need to complicate this test; it only needs to test the
behaviour of `wait_for_match` when it doesn't find a match.

Also check the exit status of `stbt-run` (without the `&&`, the test
would have passed as long as `screenshot.png` is created).
test_wait_for_match_match_method_param_affects_first_pass: Fix false negative

The first `stbt-run`'s exit status was being ignored, which masked an
error in the call to `wait_for_match`. Furthermore, I had to disable the
second pass (`confirm_method="none"`) to get an observable difference
between the two first-pass methods, because even when the first pass
was giving an incorrect match, the second pass was correctly detecting
that it shouldn't match. This test wants to prove that with a different
first-pass method you can get an incorrect "match" verdict.
test_detect_match_reports_match_position: Cleanup: Line length & whitespace
tests: Run each test in a separate "scratch" directory

Many of these tests create files in the current working directory, like
`screenshot.png` or a `stbt-debug` directory. This change stops those
tests from polluting the directory containing the source code checkout.
It also makes many tests slightly shorter, avoiding the explicit `cd
"$scratchdir"`.
make check-pylint: Hide pygobject deprecation messages

The following warning was printed for each file analysed by pylint:

    ** Message: pygobject_register_sinkfunc is deprecated (GstObject)

This deprecated function isn't directly called by any of our code, so
there is nothing we can do about it (except maybe porting to GStreamer
1.0).