tests: expand test coverage

[Anomalocaridid]

As more and more functionality is added, and refactoring is performed, it makes it increasingly likely that untested code will eventually contain bugs.

Currently, one of the main obstacles to having as complete test coverage as is feasible is testing code that runs external commands or relies on some external state.

Fortunately, tools exist for this purpose, they just need to be implemented:

• assert_cmd seems promising for integration testing.
• insta seems like a worthwhile replacement for goldie, which is currently used for golden tests.
• test-case seems helpful for generating parametrized test cases
• mockall may also be useful