docs: add note on how to write `MPI` tests

[t-bltg]

I'm porting my Fortran - MPI codes to Julia, and the first thing that comes to my mind is how do I write MPI aware tests for my package(s) ?

I couldn't find anything relevant in the docs searching with keywords such as test or runtests.jl, so this is a proposal to enhance / standardize the docs (to be discussed !).