Make it easy to run Kubernetes node tests in upstream CIs

[jlebon]

We've added Kubernetes node testing as part of #990 in the production pipeline, but now it'd be great to have it easily runnable in upstream CI too. I think it'd be good if we could have it off by default and opt in via a comment trigger. And have it output to a separate CI commit status from the main run, but ideally still reuse the same built artifacts.