coreutils: fix test suite

* gnu/packages/patches/coreutils-skip-nohup.patch: patch taken from the git
  repository of coreutils, that makes sure tests/misc/nohup.sh does not fail
  when /dev/tty does not exist.
* gnu/packages/base.scm (coreutils): use it.
* gnu-system.am (dist_patch_DATA): add it.