Track test run statistics in CI builds

[benjamincburns]

At the moment we don't appear to be generating machine-readable test reports during CI runs, or if we are, I don't think we're ingesting them anywhere. I'm not sure what features GitHub Actions supports for this, but this report data can be used for things like

• Notifying authors of broken builds which specific tests failed in their run
• Notifying fix authors when their code fixes a previously failing test
• Tracking how frequently each test tends to fail
• Spotting thrashing/flapping tests
• Tracking trends on test performance and runtime

Things that will need to be done for this:

• [ ] Configure test suites to spit out a test report file during CI runs
• [ ] Identify tooling to ingest said test run data
• [ ] Provision said tooling
• [ ] Wire up GitHub actions to shuffle test reports over to said tooling
• [ ] Bask in the glory of proper test observability

[benjamincburns]

This may be a good approach: https://github.com/marketplace/actions/publish-unit-test-results

Note that it depends on JUnit XML test report generation, but this is a really widely adopted format, and there are ways of convincing both Jest and Mocha to produce those files.