Add test for Promise prototype property descriptor

test/built-ins/Promise/prototype/prop-desc.js

+// Copyright (C) 2016 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-promise.prototype
+es6id: 25.4.4.2
+description: Property descriptor of 'prototype' property
+info: >
+  This property has the attributes { [[Writable]]: false, [[Enumerable]]:
+  false, [[Configurable]]: false }.
+includes: [propertyHelper.js]
+---*/
+
+verifyNotEnumerable(Promise, 'prototype');
+verifyNotWritable(Promise, 'prototype');
+verifyNotConfigurable(Promise, 'prototype');