Expand test coverage of xdvdfs-core, CLI commands, and the web interface

[antangelo]

We have Python tests that form a combination end-to-end/regression test suite of (re)packing an image, and reading the result, for a variety of edge-case inputs, through the CLI interface.

This is not generally sufficient though, as there are several features and code paths, particularly in the read library, that are not exercised in the end-to-end suite. Moreover, due to the structure of the Python tests, it is difficult to test the parts of the codebase that are not depended on by xdvdfs-cli (including all of xdvdfs-web and several read interfaces).

We need ...

• a framework within xdvdfs-core for unit testing individual parts of the program, especially the read functions. Presumably this can be done through an in-memory filesystem of sorts. The write portions are largely covered by the CLI tests, but analysis will need to be done on the exact coverage to determine missing points.
• to expand coverage of xdvdfs-cli, through additional Python tests primarily, to cover commands that are currently untested at the command-line level.
• to introduce testing of xdvdfs-web by some method, that at least smoke-tests the various paths through the user interface. These tests do not need to be detailed if the other functionality is verified through the CLI tests or Rust-based unit tests.