.gitignore: .stbt-* build artifact names changed to lowercase

The .stbt-* files generated while building the gstreamer plugin are all
lowercase, so modifying gitignore to adjust.

[Committer's note: The original author (see commit 31bd0a25) hadn't
noticed this error because he was using a case-insensitive filesystem.]
stbt.conf: override user's config by using run/record explicitly

If the user has a source_pipeline set under the [run] section in their
own stbt.conf (~/.config/stbt/stbt.conf), it will take precedence over
the [global] section in $STBT_CONFIG_FILE, which is set by
tests/run-tests.sh to tests/stbt.conf. We can make sure that the correct
source_pipeline is used by putting the source_pipeline for run-tests.sh
under the [run] section.
`make uninstall` deletes libexec/stbt/*.pyc files

Before this alteration `make uninstall` failed to delete the stbt
directory under libexec because the compiled python files (.pyc) were
still present.

This change deletes any .pyc files present in libexec (as there
are likely to be more than the 2 current files in the future) so that
`rmdir $(DESTDIR)$(libexecdir)/stbt` will work as expected.
stbt-debug directory removed if test which creates it passes

test_detect_motion_with_debug_output_does_not_segfault_without_mask
creates the stbt-debug directory as part of the test but previously did
not clean it up on completion.