call stopObserving in test_createBackBuffer of Tile/Image.html to prevent side-effects, test only change

tests/Tile/Image.html

@@ -326,7 +326,13 @@
         map.addLayer(layer);
         map.setCenter(new OpenLayers.LonLat(0,0), 5);
         var tile = layer.grid[0][0];
+
+        // we're going to create a back buffer while the image
+        // is actually loading, so we call stopObserving to
+        // avoid any unexpected behavior
         tile.isLoading = false;
+        OpenLayers.Event.stopObserving(tile.imgDiv);
+
         var img = tile.imgDiv;
         var bb = tile.createBackBuffer();
         t.eq(bb.style.left, tile.frame.style.left,