Add functests to slack-annotations

[seanh]

slack-annotations doesn't have any functests: when you run make functests (which is an alias for tox -e functests) or when you open a PR and CI runs on GitHub Actions, it runs the tests in tests/functional/ but those consist of only a couple of placeholder tests:

1. There's test_help() and test_version(). These are actually real unit tests for the --help and --version command-line arguments, but nothing else in cli.py is tested.
2. And there's test_it() which is just a placeholder test.

It would be good to fill this out with proper functests for much of the functionality.

Note that slack-annotations makes HTTP requests to the Hypothesis API. We don't want it to make real HTTP requests when you run the functests. Since slack-annotations uses HTTPX to send HTTP requests we may be able to use something like pytest-httpx or respx.

Done when

• slack-annotations has a set of functests which cover a reasonable amount of its functionality (we don't enforce coverage for functests so it doesn't need to be 100%)