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 <[email protected]>
Reviewed-by: Pavel Březina <[email protected]>
(cherry picked from commit deefe9a)