t/tmetadata.py: accommodate changes to xattr.set error message.

Correct the expected error string for xattr.set (changed in
21eab6b52fcf65827e21ca33fd3349157993053c).

Signed-off-by: Mark J Hewitt <m.hewitt@computer.org>
[rlb@defaultvalue.org: adjust commit message, and include a leading
 single quote in what we're looking for, to match
 c8c1f0341c82f4abf4d3f8eed1fe4ddfa4a48493.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>