Why should we limit use of PHPUnit to test the features we built only? Some failures are often due to server side configuration errors in php.ini file. My effort here is to ensure that the server always runs under your specific consideration.
As a developer, you already expertise on how to solve a particular problem. But, productivity lies on understanding the problem exactly. Being a developer you know it how to solve only after you know what is the real trouble. So, welcome the bugs; they are your multipurpose opportunities.