t/test-meta.sh: fix xattr.set test broken by path output on error.

This was broken by 21eab6b52fcf65827e21ca33fd3349157993053c, and I
(obviously) didn't test as root before pushing.

Thanks to Mark J Hewitt <mjh@idnet.com> for the report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>