tests: add 'expo_force' tests

The new value for the ldap_pwmodify_mode option 'exop_force' is added to
existing test. A new test to illustrate the different behavior of 'exop'
and 'exop_force' is added.

Reviewed-by: Justin Stephenson <jstephen@redhat.com>
Reviewed-by: Pavel Březina <pbrezina@redhat.com>
(cherry picked from commit deefe9a)