Add golden/snapshot tests

[woodruffw]

See #55.

@ubiratansoares added some amazing acceptance tests with #97, but we can take it one step further and use golden/snapshot/"known-answer" style tests to both increase our test coverage and reduce the amount of manual test code that needs to be written.

These will complement (not replace) the acceptance tests, since explicit acceptance tests are still valuable for handling concrete cases/adding backstop tests for regressions.

insta looks like the ideal library for this: https://insta.rs/