Add automated ACME testing

[ecton]

The LetsEncrypt staging directory isn't meant to be used during CI and automated testing. pebble is the recommended solution, and my original pass through made me think it would be easy to spin up a docker container.

Unfortunately, after looking more closely, it's a more involved process involving more than spinning up a container and using it's service URL. Thus, I skipped implementing it in #95.

This issue is to add automated testing of the ACME certificate acquisition code.