I have a different view on when and what to test. I would say that a test when breaks when you fix a bug is a bad test. A test that never finds a bug may be a bad test or a good test.
A lot of people do TDD with the idea that 100% test coverage is something to shoot for in itself. I am not in that camp. To me, contract oriented design and testing go hand in hand. You don't want to test every possible behavior because your view on the behavior may be wrong and there are legitimate areas you want to reserve the right to change your mind without breaking your tests,
Instead you want to test guarantees. What do you promise? Why? What are the corner cases there you need to check? Get those tested. You will usually find that results in high test quality and coverage, but not 100%, and that fixing bugs rarely breaks tests, that tests which break are showing you bugs.