This commit makes sure we can properly run the tests in a headless
environment, i.e. they don't mess with the user's X display or their
session bus. The latter is also needed for parallel tests as they fail
to simultaneously own the same name on the session bus.
Replaced the "xvfb-run" meson option with the "headless" option, which
is more intuitive (and also more correct, since we now also require
`dbus-run-session` to run the tests, not only `xvfb-run`).
Finally, note that we need a version of `xvfb-run` that supports the
`-d` (`--auto-display`) option. The problem with `--auto-servernum`
which is also regularly used, is that it doesn't shut down cleanly,
returning a non-zero exit code, wich makes the test fail.
Fixes https://gitlab.gnome.org/GNOME/gimp/-/issues/5078
0352559efa