tests: Add test showing default marshaller override
tests: Add test showing default marshaller override

https://bugzilla.gnome.org/show_bug.cgi?id=769076

johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib