Wrapping the gnome-shell call with `dbus-run-session` is *mostly*
enough for testing, but not quite.
When running without gnome-session, the wayland display is not
propagated to the D-Bus daemon, so any app that is D-Bus activated
still opens in the host session.
This can be worked around by using a specific wayland-display name
and exporting it to the `dbus-run-session` environment up-front.
At this point the invocation is finicky enough to justify another
convenience script. This also gives us a place to expose some
useful features for testing, like forcing a right-to-left layout
or simulating the greeter.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2935>
