Test idea: check number of ranks

[giordano]

We should probably have a test to check that the number of ranks of a job corresponds to the expected number.

A possible way to implement the test is:

• gather the comm sizes and ranks and do some consistency checks (e.g. length of these vectors is same as comm size and all ranks are there)
• in

  MPI.jl/test/runtests.jl

  Line 81 in 042ddc2

  cmd(n=nprocs) = `$(mpiexec()) -n $n $(Base.julia_cmd()) --startup-file=no $(joinpath(testdir, f))`

  set an env var with the number of expected processes that we can check against in the collected ranks and sizes

This came up in #899 where a broken mpich build was causing MPI jobs to have the wrong number or ranks.