CVE-2021-3670 tests/krb5/test_ldap.py: Add test for LDAP timeouts

We allow a timeout of 2x over to avoid this being a flapping test.
Samba is not very accurate on the timeout, which is not otherwise an
issue but makes this test fail sometimes.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14694



Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>