Merge branch 'sg/test-must-be-empty'

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'