tests: port to testing in Linux /tmp

* tests/selnx01.at: Discard restorecon output,
as it contains "Warning no default label for ..."
when run in Linux tmpfs on Fedora 40.

tests/selnx01.at

@@ -35,7 +35,7 @@ ln -s file dir/link
 
 getfattr -h -d -msecurity.selinux dir dir/file dir/link > start
 
-restorecon -R dir
+restorecon -R dir >/dev/null
 chcon -h --user=system_u     dir
 chcon -h --user=unconfined_u dir/file
 chcon -h --user=system_u     dir/link