Add test coverage to xdvdfs-cli commands

[antangelo]

Add comprehensive test coverage for xdvdfs-cli commands. This will require refactoring the commands to be modular, such that the underlying filesystem can be swapped out for a test double (sector linear, in-memory, and/or some other null filesystem if applicable) and input/outputs can be stubbed and verified against.

This should also include unit tests against command line parsing. The ultimate goal of this issue is to achieve similar coverage as the Python end-to-end tests on a component level within the CLI crate.