checking if 8769 covers everything regarding partial datatypes

[markalle]

@bosilca recently made https://github.com/open-mpi/ompi/pull/8769 and I'm not yet sure it's handling everything with the partial datatypes.

It's also possible I'm misusing the calls, so this is a bug report for continued investigation/discussion.

I made a testcase below that's based off the partial.c that George checked in. In unpacks 3 ints a few different ways and doesn't always get the right results.

Testcase: https://gist.github.com/markalle/eb0e67c5135c63c1ceac857de1ae3202

% cd test/datatype
% cp partial2.c partial.c
% rm -f partial
% make partial
% mpirun -np 1 ./partial