Add a test for Object.prototype.toString with Intl.RelativeTimeFormat objects.

test/intl402/RelativeTimeFormat/prototype/toStringTag/toString.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.prototype-@@tostringtag
+description: >
+    Checks Object.prototype.toString with Intl.RelativeTimeFormat objects.
+info: |
+    Intl.RelativeTimeFormat.prototype[ @@toStringTag ]
+
+    The initial value of the @@toStringTag property is the string value "Intl.RelativeTimeFormat".
+features: [Intl.RelativeTimeFormat]
+---*/
+
+assert.sameValue(Object.prototype.toString.call(Intl.RelativeTimeFormat.prototype), "[object Intl.RelativeTimeFormat]");
+assert.sameValue(Object.prototype.toString.call(new Intl.RelativeTimeFormat("en")), "[object Intl.RelativeTimeFormat]");