ci: run integration tests

[redshiftzero]

In #72 we added some integration tests. It would be nice to run these integration tests in CI, at least nightly (but if fast, also fine to do on PR).

[conorsch]

There's a bit of flakiness still that I want to shake out via local runs, before it's ready for always-on in CI. But definitely agree that we want the assurance of regular runs in CI!