Adding code coverage to the CI process can be a good way of ensuring that new code doesn't decrease the coverage of tests. There are tools that integrate with GitHub that allow you to upload your coverage report and it will provide a summary of how the coverage changed between two commits.

CodeCov is one example that I saw on another repository and it would likely suit our needs perfectly. We would just need to update the tests to run with coverage enabled and to upload the results and we would get the benefits of code coverage tracking.

It is free for open source to use so we wouldn't have to pay anything and can always remove it if it doesn't work well.