MPI test hang on `mpiexecjl` with SLURM

Testing MPI v0.20-dev (master) branch on Linux server via SLURM using `salloc` and launching Julia as `julia --project -e 'using Pkg; Pkg.test()'` via `srun -n1 ./launch_julia.sh` hangs on https://github.com/JuliaParallel/MPI.jl/blob/9a1dd861213a312fb8c16c28147d90664995ae6f/test/runtests.jl#L14-L18
although making sure that the number of processes used is consistent (`salloc -n`>=1, `srun -n1`, `export JULIA_MPI_TEST_NPROCS=1`), `ENV["SLURM_NTASKS"]` returning the expected number. Fix #559 does not yet solve the issue.