Add smoketest (package installed, keyring/repo files in place)

[rocodes]

Closes #8

• Add test that keyring/repo files are in place and rpmdb contains key
• Add smoketest make target that supports containerized test
• (WIP) add smoketest to CI
• Update README to explain rpm key id and link to rpm release management docs

[legoktm]

Two small comments about the tests, but overall looks good!

[legoktm]

Unless these need to be reused, the tests would be easier to read if the commands were inlined where they get invoked.

[rocodes]

(reuse it a bit checking multiple different packages, but made it clearer what's happening at invocation)

[rocodes]

Thank you for the feedback, if only I had pushed my extra changes bit min sooner! I've added support for running the tests in the container, addressed your comment re test failure, and (wip) adding github action support, although still figuring that part out.

I went back and forth about making the build-rpm and smoketest separate github actions, and now I'm inclined to make them separate, so that it's clear what part is failing. It's all kind of overkill for a repo that will hardly ever be updated. opinions welcome.

[rocodes]

(The way I'm trying to shove systemd into the fedora container is unsurprisingly not ideal: System has not been booted with systemd as init system (PID 1). Can't operate. 🙃 I'll look into this tomorrow)