Use multiple threads for running tests in CI

[apaszke]

No description provided.

[oxinabox]

For reference, right now Hosted Github Actions runners are always:

• 2 core for Windows/Linux
• 3 core for Mac

https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources

The code here gives some future proofing against that changing.
And would also allow private runners, that were different.

[apaszke]

Yeah I know the specs are public, but I want to avoid eyeballing them. This seemed like a more robust solution.

The downside is that we might be getting some space leaks, because the jobs seem to be running out of memory now...