Follow up to #324

Ideally we would do a test_matrix step and then a test step that depends on test_matrix, so that for branch protections we can require test independent of the Node versions we are using.

See this for reference: https://github.com/octokit/core.js/blob/393caf31b2e802ea9be59aa84f90d64f98953fdb/.github/workflows/test.yml#L32