You use tests to drive out your design, hence the term test driven game development.
If you have a different code base then your tests will be different because your design will be different. At most what I've put up will give you a decent idea of what you might want to test and potentially how to go about testing it.
If you're trying to write tests in order to test existing code you may find it in some cases to be somewhat difficult, but this is only because the code you currently have is badly designed, it's not modular or reusable.
Some things are more difficult to test than others, for example, in my code I've got an interface called icontrollable, which I mock out in my tests and use as a substitute for keyboard interaction so that the relevant bits of code in my walking engine can be tested. This also has the side effect of having a better design since my movement engine has now been separated from my keyboard logic.
Some of this might make more sense if I were to write the actual code for it to make the tests I've written pass.
edit: I've written the code needed to make the tests pass.
