chore(ci-tests): split tests across multiple machines

[erezrokah]

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Summary

Redo of #4139 with better load balancing

Todo

• Add a section in our contributing guide explaining this setup

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[ehmicky]

The speed benefit is really good: 18 minutes to 9 minutes! 🎉

[erezrokah]

Looks like some tests are failing randomly, so I'll need to figure it out.

Also, once the PR is ready we'll need to update branch protection rules to match the new status checks names

[erezrokah]

I used concurrency as I was getting out of memory errors in CI.

AVA itself runs using serial in their CI, so we should consider doing the same, see https://github.com/avajs/ava/blob/ada1a4f8876ad7b2476440609691e301a02f3951/test/snapshot-workflow/README.md#serial-execution

• concurrency - number of parallel processes to spawn (a process is spawned per test file)
• serial - number of parallel tests within the same file

[erezrokah]

This PR is ready. We'll need to update the branch protection rules to be able to merge it.

[ehmicky]

I am wondering whether YAML aliases could be used to avoid the repetitions?

[erezrokah]

actions/runner#1182 🙃

[ehmicky]

Too bad :(
Let's keep the repetition then 👍