Well the thing about testing with Haskell, the type system ensures that you're not going to have the most common bugs. If there's an error, it's probably an error in your logic or you did some math wrong or something like that. So testing, you'd basically write tests for your mental model of how the program should operate. And because of the type system you're probably going to use unsafe for a lot of test code.

Clearly the type system is comprehensively self documenting too.