Improve test coverage for moz_kinto_publisher

[jschanck]

The moz_kinto_publisher script depends on a few external services, so it is difficult to test manually. On top of that, the dev environment doesn't have write access to a Kinto server, so some parts of moz_kinto_publisher only get tested in staging. We should write some new tests with simulated services to get full coverage of the script in CI.