Bug 8148: (regression tests) C4::Auth_with_ldap needs unit tests

This followup provides some unit tests for C4::Auth_with_ldap. It implements
regression tests for this bug's changes too.

The aim was to cover all use cases for checkpw_ldap from the first line up to
those touched by this bug's patches. It should serve as a start for having
unit tests for the LDAP thing.

Having said that: no excuses for not providing regression tests for LDAP from now on :-/

Regards
To+

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>