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.
When setting up a binding between GSettings and GObject properties the
CallsSetting used to set the value from the GSetting to the property and
back to the GSetting.
While the value was still the default value it was marked as non default
because it had explicitly been set without any user interaction.