ci: shard the test run on mac os X
This should comes with some benefit:
- spread the load across more runner,
- reduce the real-time CI run,
- reduce the "retry" run when we need them.
We start with the Mac jobs, but that would be tremendously useful for Windows
too.
For linux, we need to reduce the startup overhead for this to be worth it.
Building smaller image and speeding up clone should help with that.
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