The First Rule of Test Code
Software is kind of cool in that you can write programs that verify that your other programs work correctly. These testing "meta programs" tend to get short-shrift though, because it's not like the code is actually part of the shipping product. So who cares about cleanliness or good style or all that stuff?
I'd propose that a good rule (or at least guideline) for test code is:
Treat it like production code.
That means things like:
The main justification is that the tests need to be correct and maintainable too, and all the things we've learned about good code contribute to those objectives. A large body of unmaintainable tests starts to become a liability rather than an asset. The one concession I'd make is that tests pretty much never require their own tests, because then you have a problem of infinite recursion.