tests: Unique test path for /string-visitor/output Newer GLib's want unique test paths, and thus moan at dupes. (Seen on Fedora 23 which has glib 2.46) Uniquify the paths. Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>