GitHub Actions based CI

[petemounce]

First commit is just a very skeleton workflow.

This PR is really starting as just an experiment. I'm pretty familiar with GHA, and the experiment part will really be figuring out how to interact with the Make-based build in a nice UX way but also get nice, fast, modern, all-platforms-we-want, not-as-rate-limited-as-travis CI.

If this turns out to be quite easy, then my approach would be to get to parity with all the things Travis does today, before doing anything that Travis does not do that we might also want. I'd want to minimise the amount of time there are 2 CIs in play.

That means changing the in-repo build-automation as little as possible, and if there are things necessary to change, then Travis must stay green. At all points, Travis outcome should agree with GHA outcome. If GHA differs, GHA is probably not quite there yet.

Checklist

• make test-all (UNIX) passes. CI will also test this
• unit and/or integration tests are included (if applicable)
• documentation is changed or added (if applicable)

Related: #649

(I don't think I will aim at everything that #649 wants, like researching other platforms - just getting off Travis).

[petemounce]

@aelsabbahy I think for this to start to work, you may need to do some settings changes within the repository.

I think for a contribution from a fork to run, you'd need to allow the (dangerous) fork pull request workflows permission. I advise against that; it's safer to only allow trusted contributors run CI for a tonne of reasons.

I think I can probably iterate on this in a not-fork repository that I own, then bring it back here.

[petemounce]

Note nuance to integration tests being run. #856 (comment)

[dklimpel]

I have been working on this a bit and experimenting. The biggest problem seems to be getting the centos7 images to run in github. These use systemd and the runability seems to be blocked by github.

I noticed that the jobs do not run ideally. IMHO the linitng should be removed from the test and run before the tests. But the linting is also broken and is not executed at all.