tests: Don't use g_assert_nonnull yet
tests: Don't use g_assert_nonnull yet

So that we don't require glib 2.40.

Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf,Distrotech/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,Distrotech/gdk-pixbuf