test: cancel old unit tests when a new PR commit is pushed

[kdmccormick]

Based on:

• https://docs.github.com/en/actions/using-jobs/using-concurrency#example-using-concurrency-and-the-default-behavior
• https://stackoverflow.com/questions/68418857/how-to-cancel-existing-runs-when-a-new-push-happens-on-github-actions-but-only

If this works well, we can add to other workflows and other repos.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[timmc-edx]

Ah, very nice!