I don't think testing is close to useless, I think it has a lot of value but trying to prove general correctness with your test cases is a fool's errand. Adding

I don't think testing is close to useless, I think it has a lot of value but trying to prove general correctness with your test cases is a fool's errand. Adding test cases intelligently for tricky cases in the specification and/or implementation, hard-to-hit error conditions, or adding tests for bugs you previously fixed, etc is perfectly reasonable without doggedly chasing 100% coverage or 100% correctness. And yes I'm aware of formal methods, and agree they seem like the better way to go for proving.