Test that Number.prototype.toFixed.length is 1 (#826)

Add a test to confirm that the length member of
Number.prototype.toFixed is 1.

test/built-ins/Number/prototype/toFixed/length.js

+// Copyright (C) 2017 Michael "Z" Goddard. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-number.prototype.tofixed
+es6id: 20.1.3.3
+description: >
+  Number.prototype.toFixed.length is 1.
+info: >
+  Number.prototype.toFixed ( fractionDigits )
+
+  17 ECMAScript Standard Built-in Objects:
+    Every built-in Function object, including constructors, has a length
+    property whose value is an integer. Unless otherwise specified, this
+    value is equal to the largest number of named arguments shown in the
+    subclause headings for the function description, including optional
+    parameters. However, rest parameters shown using the form “...name”
+    are not included in the default argument count.
+
+    Unless otherwise specified, the length property of a built-in Function
+    object has the attributes { [[Writable]]: false, [[Enumerable]]: false,
+    [[Configurable]]: true }.
+includes: [propertyHelper.js]
+---*/
+
+assert.sameValue(Number.prototype.toFixed.length, 1);
+
+verifyNotEnumerable(Number.prototype.toFixed, "length");
+verifyNotWritable(Number.prototype.toFixed, "length");
+verifyConfigurable(Number.prototype.toFixed, "length");