Reach 75% test coverage

Before we can put out an 0.0.1 release, we should reach 75% test coverage throughout the codebase: locally in each crate, and throughout the entire workspace.

Right now our worst crate has 0% coverage, and the workspace overall has around 68% coverage.

This will require refactoring, so I think we need to do it early, before we're locked in to hard-to-test designs.

These crates currently have coverage under 80%. The ones with 🚒 have 0% coverage.

• arti 🚒
• tor-chanmgr
• tor-circmgr
• tor-client 🚒
• tor-config
• tor-decompress
• tor-dirclient
• tor-dirmgr
• tor-netdir
• tor-proto
• tor-rtcomapt

Because tor-rtcompat (#75 (closed)) and tor-circmgr (#50 (closed)) need significant refactoring, I think it doesn't make sense to focus on them right now; similarly with tor-decompress, which can probably be removed entirely (#79 (closed)).