But if you don't run it yourself, how do you know it's working? All you know is that the tests pass.

Tests can easily do more harm than good if you let them give you a false sense of security.

But surely if you have written a unit of code, you should at least know a. What valid input the code should have, b. what output the code should return, and c. What you want he code to do! If you know these things, then wouldn't it be easy enough to write tests for at least these conditions?

A couple of seconds is too long. I run a small test suite in a couple of microsecs every time I save a file, and save my file at every change. When a task is done I run all the tests.