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

The specification changed in https://github.com/tc39/proposal-intl-list-format/pull/13.

test/intl402/ListFormat/prototype/toStringTag/toStringTag.js

@@ -0,0 +1,23 @@
+// Copyright 2018 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-intl.ListFormat.prototype-@@tostringtag
+description: >
+    Checks the @@toStringTag property of the ListFormat prototype object.
+info: |
+    Intl.ListFormat.prototype[ @@toStringTag ]
+
+    The initial value of the @@toStringTag property is the string value "Intl.ListFormat".
+
+    This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
+includes: [propertyHelper.js]
+features: [Intl.ListFormat, Symbol.toStringTag]
+---*/
+
+verifyProperty(Intl.ListFormat.prototype, Symbol.toStringTag, {
+  value: "Intl.ListFormat",
+  writable: false,
+  enumerable: false,
+  configurable: true,
+});