[Test] Update all SixSpeed tests to run in separate processes.

test/js-perf-test/SixSpeed/spread/es6.js

+// Copyright 2017 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.
+
+// This benchmark is based on the six-speed benchmark build output.
+// Copyright 2014 Kevin Decker <https://github.com/kpdecker/six-speed/>
+
+new BenchmarkSuite('ES6', [1000], [
+  new Benchmark('ES6', false, false, 0, ES6),
+]);
+
+function ES6() {
+  "use strict";
+  return Math.max(...[1, 2, 3]);
+}