We know that tests are the formal verification method we have to ensure that our code is correct. And what makes sure that the test is correct? The production code, you might think, and yes, in a way this is correct, we can think of the main code as a counter balance for our tests.
The point in writing unit tests is that we are protecting ourselves against bugs, and testing for failure scenarios we really don't want to happen in production. It's good that the tests pass, but it would be bad if they pass for the wrong reasons. That is, we can use unit tests as an automatic regression tool—if someone introduces a bug in the code, later on, we expect at least one of our tests to catch it and fail. If this doesn't happen, either ...
