rushing the code doesn't help, but coming up with testing schemes is a thing that is so orthogonal to getting simple functionality going that it does kinda come after.
i still have to catch up with this new 'fuzzing' test modality i keep hearing about. i already do a lot of tests in cryptography with fresh entropy but fuzzing uses that to create mangled inputs to make sure you didn't leave gaps in your logic. it will be interesting for me when i have a little more time to put on it.