Fix ls_tests:o_flag with ZFS TMPDIR

Unlike UFS or TMPFS, ZFS sets uarch automatically whenever a file is
updated. The test must explicitly clear uarch to be portable across
filesystems. Also, it doesn't need to run as root.

PR: 215179
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D8741