Handle another possible failing output in hplx.chpl.
Merge pull request #777 from vasslitvinov/fix-hplx.chpl-.bad-mismatch-again

Handle another possible failing output in hplx.chpl.

A couple of nights ago the test

  test/distributions/dm/t8.chpl

also reported this error:

  *** FATAL ERROR: recursive failure in AMUDP_SPMDShutdown

so I am modifying PREDIFF to accept above as an acceptable
failure mode for the .bad.

While there, extend some comments.

Discussed in principle with Brad.