Fix tileMonitoringHooks test: faked-out tile was missing an unload method.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@6501 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

tests/Layer/test_Grid.html

@@ -496,6 +496,7 @@
         var tile = {
             onLoadStart: {},
             onLoadEnd: {},
+            unload: function() {},
             events: {
                 unregister: function(name, obj, func) {
                     g_unregistered[name] = [obj, func];