Run tests via GitHub Actions on pull requests

[DivineDominion]

While I'm working on this locally, I run unit tests compulsively all the time.

For PR's, we better double-check with CI like GitHub Actions.

[ikelax]

I want to work on this. Do you already have further steps in mind?

[DivineDominion]

Nothing special. There are GitHub action templates that should do the trick. For now, we should end up running swift test. That'll compile and run tests and report the result.

Once other platforms are introduced in e.g. #2 we would need to create a "test matrix". But we'll figure that out once we get there.

[DivineDominion]

@ikelax You can take liberal inspiration from https://github.com/ChimeHQ/ChimeKit -- and if the Actions work, also uncomment the "Build" badge in the README that I prepared.