Add Function.prototype.length property descriptor test

test/built-ins/Function/prototype/length.js

@@ -0,0 +1,28 @@
+// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-properties-of-the-function-prototype-object
+description: >
+  Function.prototype.length is 0.
+info: |
+  Properties of the Function Prototype Object
+
+  The Function prototype object:
+
+  [...]
+  * has a "length" property whose value is 0.
+
+  ECMAScript Standard Built-in Objects
+
+  Unless otherwise specified, the "length" property of a built-in function object has
+  the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
+includes: [propertyHelper.js]
+---*/
+
+verifyProperty(Function.prototype, "length", {
+  value: 0,
+  writable: false,
+  enumerable: false,
+  configurable: true,
+});