Moar (ok some) tests

[yaleman]

Mainly so dependabot has a chance of flagging things when an import's wrong. I broke out some internals from main.rs into a lib so I can import them, and added runs in CI to check that clap is happy (because it panics when things are wrong at runtime, but not at compile time)

Checklist

• This pr contains no AI generated code
• cargo fmt has been run
• cargo clippy has been run
• cargo test has been run and passes
• book chapter included (if relevant)
• design document included (if relevant)