Run eggsampler/acme integration tests during CI

[cpu]

Pebble originated as a piece of test code and one of the unfortunate side effects of that is that it doesn't have significant integration test coverage (and no unit tests 😱).

The https://github.com/eggsampler/acme project has nice integration tests that run against Pebble out-of-box with the unmodified docker-compose.yml that's in the Pebble repo: https://github.com/eggsampler/acme/blob/4ddd811df83288943ed699958499c4e7a32143ed/Makefile#L11-L38

Running these tests during the Pebble CI will help catch bugs sooner.