Spurios test failures on Windows for `get_position_shared`

[vchuravy]

Test Failed at D:\a\MPI.jl\MPI.jl\test\test_io_shared.jl:70
  Expression: MPI.File.get_position_shared(fh) == sum(1:sz)
   Evaluated: 0 == 3
ERROR: LoadError: There was an error during testing

https://github.com/JuliaParallel/MPI.jl/runs/6022508469?check_suite_focus=true

We require atomicity on the file handle https://github.com/JuliaParallel/MPI.jl/blob/3b8963c7fe9a04c12aaf1725aca8cb46730d3c5c/test/test_io_shared.jl#L24, but we still seem to run into non-atomic updates...

[giordano]

This occurs also on macOS: #879. I don't think we've ever seen the failure on Linux.

NOTE: the test was skipped on Windows with #665, in case someone wanted to try and run the tests and see whether this is still an issue.