Mergify 2-level checks?

**Describe the feature request**
We've recently been discussing on Matrix questions around the number and speed of checks on our PRs. I'm going through the Mergify documentation and found https://docs.mergify.com/merge-queue/two-step/, which would let us run basic checks on PRs during development and then run the full set once approved for merging. This would require rewiring how we use Mergify (for example, it needs its own branches for queued PRs), and might encourage us to switch to using merge queues.

(There were two specific issues that have come up recently:
- Mergify merging while checks were still running because of how we have our tests set up, which might be helped by this reorganization;
- the performance of the OS X checks.)