Convert mus apptests to shelltests

mojo/shell/tests/mojo_shell_unittests.cc

-// Copyright 2016 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#include <signal.h>
-
-#include "base/test/launcher/unit_test_launcher.h"
-#include "base/test/test_suite.h"
-#include "mojo/edk/embedder/embedder.h"
-#include "mojo/shell/background/background_shell_main.h"
-
-int MasterProcessMain(int argc, char** argv) {
-  base::TestSuite test_suite(argc, argv);
-  mojo::edk::Init();
-  return base::LaunchUnitTests(
-      argc, argv,
-      base::Bind(&base::TestSuite::Run, base::Unretained(&test_suite)));
-}