Enable GitLab CI automated testing

For clarity of scope, let's remove all the coding standards fixes and whatnot. Let's only add .gitlab-ci.yml in this issue. I'll still merge it even with failures; we'll fix the coding standards violations in other issues.

The merge request now only adds the test config.

That looks fine to me. Now we can fix the failures in other issues!

Merged into 8.x-1.x. Thanks! A good next step would be to get the tests running at all -- I see they currently fail hard -- in a new issue. We don't have to get them passing, just running.

Automatically closed - issue fixed for 2 weeks with no activity.