chore(ci): add GH actions

[mavogel]

A proposal to switch from Travis to GH Actions and add steps for unit, integration, and system tests. See #325

[frairon]

Awesome @mavogel, Thanks for that!
We have been meant to migrate to gh actions for some time now, especially since Travis has moved. But never had the time.
Anyway, I don't have any experience with gh actions, so from my limited view it looks good and we should just try it.

Regarding integration tests: the naming is a bit unfortunate. Those are normal unit tests and get executed with the other unit tests when calling go test ./.... The only difference is that they use goka's tester-framework and test processors on a very high level. We use the tester-framework to test applications using goka, as in one of the examples.
We don't have to treat the intergration-tests differently from the rest (and maybe we should rename the package).

Regarding system tests: we planned on replacing the flag with an ENV-variable, as that seems less flaky when goka is used as a libary using a different flag-library. Just saying, we'll probably modify the gh-actions for that, whenever we replace that behavior.

Also we're in the process of adding a github-page to goka, so we can have proper documentation on all the things that keep stacking up to be documented. (And close #325, yay)

So maybe let's just remove the integration-job for now?

Thanks again!

[frairon]

@mavogel
If you don't mind I'd like to merge that, so we can use your contribution and modify only some bits here and there.

[mavogel]

@frairon sorry for the delay. Feel free to merge, however, if I shall remove the integration-job, I can do this of course for now

So maybe let's just remove the integration-job for now?

[frairon]

no worries, I'll just tweak it after merging. Thanks!