Align output between serial and parallel h5diff

See related discussion in https://github.com/HDFGroup/hdf5/pull/5416. Output from (p)h5diff may end up in either stdout or stderr, depending on whether serial diff is used, parallel diff is used or parallel diff is used but with only 1 MPI process. Differences between the 3 cases should be aligned so that checking of testing output works as expected.