We recently had a rather short challenge without any test cases (EDIT: no longer true). This sparked some conversation in chat, as well as some comments concerning its lack of test cases.
As far as I can tell, we haven't formally set down any rules as to whether we need test cases or not. Therefore:
Should test cases be required on challenges?
(Obviously, certain types of challenges inherently don't have test cases, such as king-of-the-hill and cops-and-robbers, so I'm not talking about those)