add Reflect[Symbol.toStringTag]

see tc39/ecma262#2057

test/built-ins/Reflect/Symbol.toStringTag.js

@@ -0,0 +1,20 @@
+// Copyright (C) 2020 Jordan Harband. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+description: >
+    `Symbol.toStringTag` property descriptor
+info: |
+    The initial value of the @@toStringTag property is the String value
+    "Reflect".
+
+    This property has the attributes { [[Writable]]: false, [[Enumerable]]:
+    false, [[Configurable]]: true }.
+includes: [propertyHelper.js]
+features: [Symbol.toStringTag, Reflect]
+---*/
+
+assert.sameValue(Reflect[Symbol.toStringTag], 'Reflect');
+
+verifyNotEnumerable(Reflect, Symbol.toStringTag);
+verifyNotWritable(Reflect, Symbol.toStringTag);
+verifyConfigurable(Reflect, Symbol.toStringTag);