The documentation for gtk_widget_get_action_group
and gtk_widget_list_prefixes states that both of
these operate on all the action groups that are
'available' to the widget.
Which means: they are meant to walk up the parent
muxer chain. So do that.
Add tests to verify the expected behavior.
Closes: https://gitlab.gnome.org/GNOME/gtk/issues/1396
