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

MPI.jl/test/test_io_shared.jl

Line 24 in 3b8963c

MPI.File.set_atomicity(fh, true)

, but we still seem to run into non-atomic updates...