CI: add code coverage check

[jrfnl]

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Follow up action after #628

Conversation about this between @GaryJones and @jrfnl from #628:

Register with coveralls.io (or another service) and start recording and checking code coverage for builds.

What sort of code coverage % do we currently have?

It's one of the things I've been monitoring locally when I have been working on individual sniffs.

Can this be done within the GitHub Actions rather than using an external service to display the results?

Yes, we can use the text reporter instead of checkstyle to see the results in the action.

We'd need to investigate how we can make the build fail on a decrease in code coverage in that case though. That part is what external services do automatically (and with configurable settings).