CI: add code coverage check

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.
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>> What sort of code coverage % do we currently have?
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>It's one of the things I've been monitoring locally when I have been working on individual sniffs.
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>> Can this be done within the GitHub Actions rather than using an external service to display the results?
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>Yes, we can use the `text` reporter instead of checkstyle to see the results in the action.
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>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).