feat(CI): run linux tests with pytest-xdist

[mayeut]

This revisits #412.
With musllinux around the corner, CI times are growing quite a bit.

This does not apply to Windows/macOS (yet) for the reasons outlined in #412.

[joerick]

So the GHA timings from main are currently 37m, this is 23m. But, yeah, #768 takes the linux test time up to 60-70mins, so this is probably gonna be pretty handy.

I am considering if it would be worth being a bit more selective in #768 about which tests we run on muslinux, but it is very reassuring to know that all our tests pass on musllinux, just like manylinux. Maybe we wait and see how performance is in #768 after this is applied.

[Czaki]

for windows, it should be possible to achieve by monkeypatching installation path

cibuildwheel/cibuildwheel/windows.py

Line 98 in d5add42

installation_path = Path(nuget_args[-1]) / (nuget_args[0] + "." + version) / "tools"

To od this I suggest changing hardcoded "tools" on a global contact. For macOS I have no idea.

[mayeut]

I am considering if it would be worth being a bit more selective in #768 about which tests we run on muslinux, but it is very reassuring to know that all our tests pass on musllinux, just like manylinux. Maybe we wait and see how performance is in #768 after this is applied.

Agreed on selectivity of the tests. This might be something that can have a broader scope than just musllinux IMHO.

for windows, it should be possible to achieve by monkeypatching installation path

Eventually, I'd like to be able to run cibuildwheel locally on all 3 platforms with no unknown side-effects on all 3 platforms. This has already been mentioned a few times & will require tweaking the Windows & macOS code a bit (Linux running in docker already has only known side-effects: images being pulled). It will probably be a good idea to keep this use-case in mind when doing that.
I have some ideas for both OS but let's keep this linux only for now.

[Czaki]

The biggest problem for me is to install python locally on macOS (other than compile from source). For windows, the only side effect may be NuGet installation.

[joerick]

This is bringing the linux times back down to 34m, which is pretty good!