Fix invalid test for "does not walk the same property twice"

The previous test only worked because of a faulty implementation in our source,
once that is fixed, this test will fail, because we walk ALL own properties.

Instead, this test has been rewritten to ensure that each property is only
examined once, by looking at what property names have been used.

test/walk-test.js

@@ -143,15 +143,22 @@
 
         "does not walk the same property twice": function () {
             var parent = {
-                func: function () {}
+                func: function parentFunc() {}
             };
             var child = sinon.create(parent);
-            child.func = function () {};
+            child.func = function childFunc() {};
             var iterator = sinon.spy();
 
             sinon.walk(child, iterator);
 
-            assert.equals(iterator.callCount, 1);
+            var propertyNames = iterator.args.map(function (call) {
+                return call[1];
+            });
+
+            // make sure that each property name only exists once
+            propertyNames.forEach(function (name, index) {
+                assert.equals(index, propertyNames.lastIndexOf(name));
+            });
         }
     });
 }(this));