Investigate using path filters to reduce the number of tests we are running

[madolson]

If someone updates the README file, we run all of the tests. We should investigate using path filters, https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore, so we only have to run tests when it actually makes sense to do so. My only initial suggestion is we shouldn't run the functional tests if *.md is modified, but maybe folks have other ideas.