A test is not a proof, and you can prove something works without ever even running it. There are also properties of a system which are impossible to test, or are so non-deterministic that you a test will only fail once every million times the code is executed. You really need to just learn to prove stuff.
The most complex piece of code I have ever written, as a relevant story, took me a month to prove to everyone that it was correct. We then sent it off to multiple external auditors, one of which who had designed tooling that would let them do abstract interpretation with recursion, to verify I hadn't made any incorrect assumptions. The auditors were confused, as the code we sent them didn't do anything at all, as it had a stupid typo in a variable name... I had never managed to figure out how to run it ;P. But... they found no other bugs!
In truth, the people whom I have met whom are, by far, the worst at this, are the people who rely on testing, as they seem to have entirely atrophied the ability to verify correctness by reading the code and modeling it in some mathematical way. They certainly have no typos in their code ;P, but because a test isn't a proof, they always make assumptions in the test suite which are never challenged.
The most complex piece of code I have ever written, as a relevant story, took me a month to prove to everyone that it was correct. We then sent it off to multiple external auditors, one of which who had designed tooling that would let them do abstract interpretation with recursion, to verify I hadn't made any incorrect assumptions. The auditors were confused, as the code we sent them didn't do anything at all, as it had a stupid typo in a variable name... I had never managed to figure out how to run it ;P. But... they found no other bugs!
In truth, the people whom I have met whom are, by far, the worst at this, are the people who rely on testing, as they seem to have entirely atrophied the ability to verify correctness by reading the code and modeling it in some mathematical way. They certainly have no typos in their code ;P, but because a test isn't a proof, they always make assumptions in the test suite which are never challenged.