Add a test for Intl.Segmenter.prototype[@@toStringTag].

This was added to the specification in https://github.com/tc39/proposal-intl-segmenter/pull/29.

test/intl402/Segmenter/prototype/toStringTag/toStringTag.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 the @@toStringTag property of the Segmenter prototype object.
+info: |
+    Intl.Segmenter.prototype[ @@toStringTag ]
+
+    The initial value of the @@toStringTag property is the string value "Intl.Segmenter".
+
+    This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
+includes: [propertyHelper.js]
+features: [Intl.Segmenter, Symbol.toStringTag]
+---*/
+
+verifyProperty(Intl.Segmenter.prototype, Symbol.toStringTag, {
+  value: "Intl.Segmenter",
+  writable: false,
+  enumerable: false,
+  configurable: true,
+});