GtkDrawingArea: Set an accessible role

And update the a11y tests to match. Pointed out in
https://bugzilla.gnome.org/show_bug.cgi?id=721040

gtk/gtkdrawingarea.c

@@ -142,6 +142,8 @@ gtk_drawing_area_class_init (GtkDrawingAreaClass *class)
 
   widget_class->realize = gtk_drawing_area_realize;
   widget_class->size_allocate = gtk_drawing_area_size_allocate;
+
+  gtk_widget_class_set_accessible_role (widget_class, ATK_ROLE_DRAWING_AREA);
 }
 
 static void