"... to determine what tests you need to run, your interface isn't correctly designed."
There is a big hole in this logic. Testing should not and cannot depend on the correctness of anything else, regardless whether it is the interface design or the actual code. There is simply no dependency, we don't expect it, and we avoid it. The whole purpose of testing is to find faults with the application, from requirements to design, to coding.
If we assume that the interface design is great, or depend on that, then the entire purpose of the testing is defeated right there.