CVE-2023-0614 selftest: Use setUpClass() to reduce "make test TESTS=large_ldap" time

CVE-2023-0614 selftest: Use setUpClass() to reduce "make test TESTS=large_ldap" time

This reduces the elapsed time to 6m from 20m on my laptop.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15332
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15270

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Tue Mar 14 07:16:04 UTC 2023 on atb-devel-224

(cherry picked from commit b4a6c054ec6acefacd22cb7230a783d20cb07c05)

[abartlet@samba.org Included in the security release as this
 makes working on the large_ldap test practical by reducing
 the elapsed time taken]