Execute --runslow tests in CI

[jjallaire-aisi]

Will require that we run docker in CI.

[tadamcz]

Prior discussion: #726

I have experience with running Docker in GitHub actions. @art-dsit let me know if it would be helpful to chat about this or the CI/testing setup in general (my work email is on my employer's website). Equally, if you already have a clear plan of action / just want to move ahead on your own, that's absolutely fine. Don't let me block you. :)