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

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