Intl.RelativeTimeFormat: Add a test for the return value of supportedLocalesOf()

test/intl402/RelativeTimeFormat/constructor/supportedLocalesOf/result-type.js

+// Copyright 2018 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-Intl.RelativeTimeFormat.supportedLocalesOf
+description: Verifies the type of the return value of Intl.RelativeTimeFormat.supportedLocalesOf().
+info: |
+    Intl.RelativeTimeFormat.supportedLocalesOf ( locales [, options ])
+includes: [propertyHelper.js]
+features: [Intl.RelativeTimeFormat]
+---*/
+
+const result = Intl.RelativeTimeFormat.supportedLocalesOf("en");
+assert.sameValue(Array.isArray(result), true,
+  "Array.isArray() should return true");
+assert.sameValue(Object.getPrototypeOf(result), Array.prototype,
+  "The prototype should be Array.prototype");
+assert.sameValue(Object.isExtensible(result), true,
+  "Object.isExtensible() should return true");
+
+assert.notSameValue(result.length, 0);
+for (let i = 0; i < result.length; ++i) {
+  verifyProperty(result, String(i), {
+    "writable": false,
+    "enumerable": true,
+    "configurable": false,
+  });
+}
+
+verifyProperty(result, "length", {
+  "writable": false,
+  "enumerable": false,
+  "configurable": false,
+});