minor test improvements

[Jim Meyering]

Here are a few test-improving patches:

  1/6 tests: minor improvement of sc_tight_scope rule
  2/6 tests: convert common root-build test failure to a "skip"
  3/6 tests: preserve-gid: remove useless use of "env"
  4/6 tests: preserve-gid: don't chown temporary PATH dir to a nameless UID
  5/6 tests: don't ever leave a backgrounded "sleep 10m" process
  6/6 tests: avoid spurious parallel failure due to temporary disk full