How to Stop a Hacker From Using Voting Machines

Voting machines were all tested and found not to have any problems. Could a hacker create a system where when you tested the voting machine it tested right, but when you used it, it didn't? Is that possible? Let me tell you how i would do it if i were hacker. I'm not suggesting this happened. I just want to know what would stop hit from hacking. The point is that the main media has convinced democrats that there is a hundred % nowability about whether our systems were were good. That seems like such a naive assumption that it has to be cognitive distance.