Rather use gapplication service. This makes the flow similar to what
Chatty does and makes us hold/release rather than having an application
window.
It also allows us to drop the --daemon.
Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/760>
gtk_test_init() ends up calling gtk_init() which tries opening
the default display. CI jobs sometimes seem to trip over this
even when run through xvfb-run. Therefore we use g_test_init()
instead.
