Add test for making private symbols non-enumerable

BUG=chromium:664411

Review-Url: https://codereview.chromium.org/2498963002
Cr-Commit-Position: refs/heads/master@{#40950}

test/mjsunit/regress/regress-private-enumerable.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+class A {}
+class B {}
+Object.assign(B, A);
+assertEquals("class B {}", B.toString());