Add CI job to run tests sequentially

[AA-Turner]

Add a CI job to run all tests sequentially. This takes ~30m, so might be better as a buildbot worker?

Open questions:

• How close to the release-tools check should we make this? Currently I've mirrored it quite closely, but we probably ought to be closer to the other common test configurations.
• Should the job be required, if on CI? Currently I don't think it should be.
• Should we add a make target for sequential testing?

A

[hugovk]

Thanks for this! Will take another look after the beta is out :)

This takes ~30m, so might be better as a buildbot worker?

Yeah, maybe. The current slowest job is free-threaded Windows at about 25m, so this new one would become the slowest. And hopefully we can improve the Windows one too.