Verify shiny app behavior with tests

[nmdefries]

Testing shiny apps is hard in general. There are some snapshot-based tests to check that behavior is stable. The app behavior is not finalized yet so we'll hold off on that for now, but unit tests and assertions/checks in-code would be useful. The app doesn't use a tone of functions at the momen, so we also need to refactor it to isolate testable code.