> You cannot make edge cases and strange unique behavior happen in a controlled environment.

Sure you can. Go iterate through the GNU Coreutils bug tracker, find weird bugs, create test cases, feed them to your implementation. Bonus points if you can find an existing test suite (ex. xfstests turns out to be good for testing arbitrary filesystems). Granted, there will always be edge cases that you don't catch until they show up live in prod, but you can hit a chunk of the space without that.