Investigate using PR annotations

[duckinator]

By having pytest generate junit-compatible XML reports in CI, we can theoretically have PRs show what tests failed without having to even pulling up the Cirrus CI website.

• https://cirrus-ci.org/examples/#unittest-annotations
• https://medium.com/cirruslabs/github-annotations-support-227d179cde31
• https://stackoverflow.com/a/30230895
• https://docs.pytest.org/en/latest/how-to/output.html#creating-junitxml-format-files