Bug 12831: (RM followup) regression test

Bug 12831: (RM followup) regression test

checkpw_ldap should return 0 if it is not an anonymous bind, and authentication
fails. This is better explained on the bug comments. This is just a regression
test for the revised functionality.

To test:
- Run
  $ prove t/db_dependent/Auth_with_ldap.t
=> FAIL: it fails because C4::Auth_with_ldap doesn't match the expected behaviour
- Apply the bugfix from Martin
- Run
  $ prove t/db_dependent/Auth_with_ldap.t
=> SUCCESS: tests now pass.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>