Intl.RelativeTimeFormat: Add a basic test for the functionality of supportedLocalesOf().

This is based on test/intl402/NumberFormat/supportedLocalesOf/basic.js.

test/intl402/RelativeTimeFormat/constructor/supportedLocalesOf/basic.js

+// Copyright 2018 Google Inc., 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: >
+    Tests that Intl.RelativeTimeFormat has a supportedLocalesOf property,
+    and it works as planned.
+features: [Intl.RelativeTimeFormat]
+---*/
+
+assert.sameValue(typeof Intl.RelativeTimeFormat.supportedLocalesOf, "function",
+                 "supportedLocalesOf should be supported.");
+
+const defaultLocale = new Intl.RelativeTimeFormat().resolvedOptions().locale;
+const notSupported = 'zxx'; // "no linguistic content"
+const requestedLocales = [defaultLocale, notSupported];
+
+const supportedLocales = Intl.RelativeTimeFormat.supportedLocalesOf(requestedLocales);
+assert.sameValue(supportedLocales.length, 1, 'The length of supported locales list is not 1.');
+assert.sameValue(supportedLocales[0], defaultLocale, 'The default locale is not returned in the supported list.');