The tests must be isolated, so that they can be run independently. If something is so inanely simple that you're sure it will work, then don't bother to write a test for it. If it turns out you were wrong and there was a bug, then write a test for it for future use. There are two types of tests. The programmers write the first tests to unit test all of their code.
Customers write the second tests.
These are functional tests to verify that their stories will work in the system implementation. This is an area I find sorely lacking in any sort of an explanation or example. Customer testing is supposed to be aided by a dedicated tester on an XP project. Pair Programming Pair programming is exactly what the name would indicate. It is two people sitting together at the same workstation and creating code.