A lot of tests, especially unit tests, are just change detectors and get updated/go away when change happens, that is just WAI. It is fairly hard to write non change detection tests, it requires for you to really reason abstractly about the contract of your module, or to write integration tests that are moving a bunch of things at once.
small, fine-grained black box tests can be really good for this. in my last project, a type checker, the vast majority of the test suite was code snippets and assertions about expected errors the checker needed to catch, and it was an invaluable aid when making complex changes to the implementation.
Anything that transforms or processes text, like a compiler or type checker, is pretty easy to test. You get into trouble with user interfaces, however.
