tests/animated-resizing: Use gtk_window_default_size() instead

This will actually make it resize on-demand, as the default size is the
size used when computing the window size when the size is not fixed.

tests/animated-resizing.c

@@ -123,7 +123,7 @@ on_frame (double progress)
       window_height = HEIGHT + jitter;
     }
 
-  gtk_widget_set_size_request (gtk_window_get_child (GTK_WINDOW (window)),
+  gtk_window_set_default_size (GTK_WINDOW (window),
                                window_width, window_height);
 
   gtk_widget_queue_draw (window);