Run testsuite with GitHub Actions

[pprkut]

It would be nice if we could run the testsuite with GitHub Actions. The challenge here is that (by default) GitHub Actions run on ubuntu. Running on Slackware would require a custom runner being hosted somewhere.
We can investigate, however, if we can get all the required dependencies available on a ubuntu system (pkgtools, primarily).
Another alternative would be trying to run the testsuite inside a Slackware docker container.