tests/tree_model_iterator: Fix CRITICALs as !model
Kjell noticed at line 118 and the others creating paths from iterators:
Gtk-CRITICAL **: 10:21:51.432: gtk_tree_model_get_path: assertion 'GTK_IS_TREE_MODEL (tree_model)' failed
These criticals were because the paths were created from iterators not associated with a model. Make them use the same 'dummy' model as others.