- index.php is the TestSuite. It loads all testcases from the phpUnit directory, and executes all test_* functions from all classes that implement PHPUnit_TestCase.
- button.php contains all tests for the button control class. I think this is the most simple class GO has implemented, but there is a lot of tests for this class - and some tests for invalid parameters aren't implemented yet (because the class itself doesn't check if action contains "-characters and so on).
I know that it isn't simple to get yourself to write testcases or documentation or something similar which has no direct seeable effect on the project, but if correctly done it can make your life easier.
And: If you write your testcases first, that means that it is "documented" what the class should do, and when you finished the implementation you do not need to test it, because you've already done that