Develop + roll out testing framework #5

The work thus far has been largely experimental but now that we feel relatively confident about the direction of the CLI, it's time to start adding tests to reinforce functionality and catch regressions. Deno has a built-in test framework and Cliffy supports injecting tests; the question marks will be around what needs to be tested in the CLI versus what is functionality that's happening API-side and should be tested through there.

(thx @elauffenburger)