Not all forms of testing consist of separate test suites like unit tests. Assertions, contracts, encoding constraints in elaborate type systems and having them

Not all forms of testing consist of separate test suites like unit tests. Assertions, contracts, encoding constraints in elaborate type systems and having them checked at compile time are also ways of coding repeatable tests. And if it's some poorly coded legacy application or something that's mostly wrapping a proprietary black box, working around other peoples bugs, and things like that then system or integration tests may be easier than attempting to mock a gordian knot's tangles. Unit tests work best for properly encapsulated pieces of code, utility classes, libraries and such.