[ldap] an empty 'user-base-dn' disable the user importation process,
The same semantic will be used for group. We start by using it for user for the
sake of consistency.
[ldap] Major test refactoring
Mostly fix tests (previous refactoring), but also refactor a bit the base class
to have better/simpler interaction with the slapd server.
[test/ldap] have RQL2LDAPFilterTC use its own database
I do not know what RQL2LDAPFilterTC is about and why it does this strange reuse
of LDAPUserSourceTC pre_setup_database but it is certainly a good idea to have it
use a dedicated database-cache.