Those classes don't change. If there's is a bug in them, you'd find it during your normal development and testing activities (unless it's an obscure one, but then the unit test wouldn't find it either).
The OpenJDK project is sure to have plenty of unit tests for those classes that are way more elaborate than anything you would ever come up with.
That article talks about classes you write, or classes that might differ between development and production (maybe using a test service vs. a real service), or classes that might not be present in a development or test environment (like an HttpServletRequest if you're writing
servlets). It does not apply to these objects that are the same everywhere.