CI improvement suggestion: only run tests that test changed code

[naseemkullah]

e.g.: https://medium.com/@stipsan/best-kept-jest-secret-testing-only-changed-files-with-coverage-reports-3affc8b4d30f

thoughts?

[dyladan]

This is something I've considered in the past, but it's really tough to do effectively since a change to one package may affect behavior in another. This particular blog post also uses jest, which we're not using. We could consider using lerna to test packages that have been changed (and their dependents). Generally I only test one package at a time locally anyway so its only an issue in CI.