Test fixes.

* sg/test-must-be-empty:
  tests: use 'test_must_be_empty' instead of 'test_cmp <empty> <out>'
  tests: use 'test_must_be_empty' instead of 'test_cmp /dev/null <out>'
  tests: use 'test_must_be_empty' instead of 'test ! -s'
  tests: use 'test_must_be_empty' instead of '! test -s'