My fear with doing that is that if the functions file is separated from the tests file, then it becomes difficult to see at a glance what the function needs to

My fear with doing that is that if the functions file is separated from the tests file, then it becomes difficult to see at a glance what the function needs to do in order to make the test pass. That said, I agree that what you're supposed to do could be more clear -- would you care to open an issue so we can discuss it on the repo?