When moving from gtk_container_forall to the widget dom
api, we are now iterating over all children of the listbox,
including headers, separators, etc. So, skip everything
that is not a listboxrow, to make the tests work again.
This test was relying on gtk_container_forall returning
the visual (ie sorted) order of children, while iterating
with the widget dom api gives the insertion order.
Instead of using gtk_container_forall, use
gtk_list_box_row_get_index to reconstruct the visual
order.
