Parallelize test/mjsunit/es6/tail-call-megatest.js in order to avoid timeouts on slow architectures.

test/mjsunit/es6/tail-call-megatest-shard8.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.
+
+// Flags: --allow-natives-syntax --harmony-tailcalls
+// TODO(v8:4698), TODO(ishell): support these cases.
+// Flags: --turbo --nostress-opt
+
+try {
+  load("mjsunit/es6/tail-call-megatest.js");
+} catch(e) {
+  load("test/mjsunit/es6/tail-call-megatest.js");
+}
+
+run_tests(8);