This test checks for a defect found in the 4.0 releases of MPICH.

An incorrect element size is derived in some cases and leads to
segmentation faults subsequently.

The bug is tracked here:

<https://github.com/pmodels/mpich/issues/6341>

A corresponding minimal patch is part of this distribution at

config/checkpatch/mpich_4.0.x_ddt_transfer.patch

The issue is resolved in newer versions of MPICH and not present in
older releases.
