We're hitting GitHub concurrency limits (organization wide limit of 60
jobs), and while we could move to paid runners with high concurrency
limits, I'd prefer to stay on the free runners and some of these jobs,
e.g., `test-system`, require GitHub runners.
This moves a bunch of our extended testing behind a label, e.g.,
`test:extended` or `test:system`, and only runs them on `main` by
default.