Test suites don't verify correctness. They just ensure that you haven't broke something so bad the specific instances that the tests assert have turned into a failure. You can have a factorial function and more likely the test cases will only be a few numbers. Which does not guarantee correctness as someone who know about the test cases can just put a switch and return the correct response for those specific cases.

The compromise is worth it in traditional coding, because someone will care about the implementation. The test cases are more like the canary in the coal mine. A failure warrants investigations but an all green is not a guarantee of success.

Regardless that is why most people are moving faster than writing by hand