intl: improve coverage for Intl.NumberFormat

Add tests for Intl.NumberFormat.prototype[@@toStringTag].

test/intl402/NumberFormat/prototype/toStringTag/initial-value.js

+// Copyright (C) 2018 Ujjwal Sharma. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-intl.numberformat.prototype-@@tostringtag
+description: >
+  Initial value of Intl.NumberFormat.prototype[@@toStringTag] is "Object".
+---*/
+
+assert.sameValue(Intl.NumberFormat.prototype[Symbol.toStringTag], 'Object');
+assert.sameValue(
+  Object.prototype.toString.call(new Intl.NumberFormat()),
+  '[object Object]'
+);

test/intl402/NumberFormat/prototype/toStringTag/prop-desc.js

+// Copyright (C) 2018 Ujjwal Sharma. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-intl.numberformat.prototype-@@tostringtag
+description: >
+  Tests that Intl.NumberFormat.prototype[@@toStringTag] has the required attributes.
+includes: [propertyHelper.js]
+---*/
+
+verifyProperty(Intl.NumberFormat, Symbol.toStringTag, {
+  writable: false,
+  enumerable: false,
+  configurable: true
+});