Add test for AggregateError's toString

Ref tc39/proposal-promise-any#49

test/built-ins/AggregateError/toString.js

@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Leo Balter. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+description: >
+  Instance reuses the toString from Error.prototype
+esid: sec-aggregate-error
+info: |
+  Ref https://github.com/tc39/proposal-promise-any/pull/49
+
+  Properties of the AggregateError Constructor
+
+  - has a [[Prototype]] internal slot whose value is the intrinsic object %Error%.
+features: [AggregateError]
+---*/
+
+assert.sameValue(
+  AggregateError.prototype.toString,
+  Error.prototype.toString,
+  'AggregateError inherits the toString method from Error.prototype'
+);
+
+var sample = new AggregateError([], '');
+
+assert.sameValue(
+  sample.toString,
+  Error.prototype.toString,
+  'toString is inherited from Error.prototype'
+);