improve propertyHelper for failure messages (tc39#1480)

Example:
Before: descriptor value should be 42 Expected SameValue(«1», «0») to be true
After: descriptor value should be 42

harness/propertyHelper.js

@@ -73,7 +73,7 @@ function verifyProperty(obj, name, desc, options) {
     }
   }
 
-  assert.sameValue(failures.length, 0, failures.join('; '));
+  assert(!failures.length, failures.join('; '));
 
   if (options && options.restore) {
     Object.defineProperty(obj, name, originalDesc);