UnitTest Study in http://www.raywenderlich.com/zh-hans/25658/%E5%9C%A8-xcode-%E4%B8%AD%E8%BF%9B%E8%A1%8C%E8%87%AA%E5%8A%A8%E5%8C%96%E6%B5%8B%E8%AF%95-12