Add automated testing using GitHub Actions

[korydraughn]

[alanking]

I'm guessing we are not going to do this for 4-3-stable, correct?

[korydraughn]

There aren't any plans for that at the moment.

This work was planned as a 5.x thing since 4.3.5 is close to moving into release testing.

[trel]

that and we're only testing backward compatibility at the moment with released software, rather than tip of an unreleased branch.