Add basic test suite for behavior and UI

[cassaundra]

I've deferred this long enough. It's time to actually make a test suite now that program is relatively stable. For the first iteration, tests should include:

• Before-and-after filesystem tests that prove tasks behave as intended (deferred)
• UI tests of stdout/stderr to make sure everything looks right
• Various tests of invalid state (invalid syntax, dependency graph, runtime error, etc.) (deferred)

The snapbox crate is well suited for some of these purposes.

[epage]

With #26, we've talked about TUI tests but haven't been able to put any time to it. Unsure if https://crates.io/crates/term-transcript can help with that which I use grudgingly to create a screenshot for https://docs.rs/winnow/latest/winnow/trace/index.html as the workflow behind it is rough (I'd like to at least have a subset of it in snapbox...)

[cassaundra]

Basic implementation in 00464c2. Will follow up with more example projects as tests as they are developed. There should still be a systematic suite that contains at least every error, pending near completion of #52.