When Vibe coding/engineering I don't think of tests in the same way as when testing human written code.

I use unit tests to "lock down" current behavior so an agent rummaging around feature F doesn't break features A and B and will get immediate feedback if that happens.

I'm not trying to match every edge case, but focus more on end to end tests where input and output are locked golden files. "If this comes in, this exact thing must come out the other end." type of thing.

The AI can figure out what went wrong if the tests fail.