coreutils: 8.30 -> 8.31, touchups

Release notes:

https://savannah.gnu.org/forum/forum.php?forum_id=9394

Touchups:
* enable tests w/musl (disable 2 gnulib tests)
* improve tests we modify: use exit code 77 to
  indicate test skipped (causing it to be reportd as SKIP)

Note: Not yet on mirrors, but can be (manually) fetched from primary