tests: Run /gsettings/keyfile test in a temporary directory
tests: Run /gsettings/keyfile test in a temporary directory

Don’t pollute the build directory with files generated by running the
test.

Note that there are still other tests in the gsettings.c test suite
which use the build directory, but fixing them is a bit more involved
than I have time for right now. This is a step in the right direction.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib