Then do you need tests to validate your tests are correct, otherwise the LLM might just generate passing code even if the test is bad? Or write code that games the system because it's easier to hardcode an output value then to do the actual work.

There probably is a setup where this works well, but the LLM and humans need to be able to move across the respective boundaries fluidly...

Writing clear requirements and letting the AI take care of the bulk of both sides seems more streamlined and productive.