Unit tests become the specification once they are written. ChatGPT can easily write unit tests from whatever your specification is before that -- such as documentation, a bunch of comments and stubs, or even a first draft of the function itself, given enough context from the rest of the project.

Unit tests are too klunky to think in. You don't prototype the specification by implementing unit tests. And you really only lay down a few critical paths even if you "write the tests first" because code paths always come up during implementation that demand more test coverage anyway.