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).

Commencez par examiner la configuration GitHub Actions du dépôt et la discussion sur la couverture dans #628. Déterminez si la couverture doit utiliser Coveralls ou un rapporteur texte dans l’action, comment le pourcentage actuel est établi et comment une diminution doit faire échouer le build. C’est terminé lorsque les builds enregistrent la couverture et appliquent le contrôle convenu en cas de diminution.

Rédigé par le modèle d'indexation à partir du texte de l'issue.