The main reason why your mock breaks later is because you refactored the code. You did the one thing tests are supposed to help you do, and the tests broke. If code was never modified, you wouldn't need automated tests. You'd just test it manually one time and never touch it again. The whole point of tests is you probably will rewrite internals later as you add new features, improve performance or just figure out better ways to write things. Mock-heavy tests are completely pointless in this respect. You end up rewriting the code and the test every time you touch it.

There are really only a few reasons to use mocks at all. Like avoiding network services, nondeterminism, or performance reasons. If you need to do a lot of mocking in your tests this is a red flag and a sign that you could write your code differently. In this case you could just make the config file location an optional argument and set up one in a temp location in the tests. No mocking required and you're testing the real API of the config file module.