test(truecmd): cover GNU-compatible exit-0 behavior
Adds the previously missing test file for the true coreutil.
Asserts nil error (exit 0) with empty stdout/stderr
regardless of arguments, matching GNU coreutils.
Refs: docs/posix2018/CONFORMANCE.md
Assisted-by: Claude Opus 4.7 via Claude Code
Signed-off-by: Xe Iaso <me@xeiaso.net>