[CI] Intermittent failure in `test_io_shared` test on macOS

[giordano]

On macOS, with any MPI library, we occasionally see a failure in test_io_shared (example):

Test Failed at /Users/runner/work/MPI.jl/MPI.jl/test/test_io_shared.jl:49
  Expression: MPI.File.get_position_shared(fh) == sum(1:sz)
   Evaluated: 0 == 10

[luraess]

We see similar behaviour in other projects using macOS CI test for shared MPI IO. Looking fwd to see if we can nail the thing down.

[giordano]

Oh, I just realised this is basically the same as #555

[giordano]

We see similar behaviour in other projects using macOS CI test for shared MPI IO. Looking fwd to see if we can nail the thing down.

I guess coming up with a reproducible example would help a lot. Random failures are hard to debug (and rr isn't even an option at all here, since it's outside of Linux).