[debugger] basic test infrastructure for new debugger test api.

test/debugger/protocol/basic-round-trip.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.
+
+var received = 0;
+
+function receive(message) {
+  var message_obj = JSON.parse(message);
+  assertEquals(3, message_obj.result.result.value);
+  received++;
+}
+
+var message = JSON.stringify({
+  id : 1,
+  method : "Runtime.evaluate",
+  params : {
+    expression: "function f() { return 2 }; 3"
+  }
+});
+
+send(message);
+
+assertEquals(1, received);
+assertEquals(2, f());