Extend LDAP tests to use LDAP servers with access restrictions.

Extend LDAP tests to use LDAP servers with access restrictions.

Add access restrictions to the LDAP test server instances.
New modes used a test parameters are ANONYMOUS, DS_MANAGER and USR_MANAGER.
ANONYMOUS can bind anonymously and access users and groups.
In DS_MANAGER the server requires authentication and will only allow
the DIRECTORY_MANAGER user to search for users and groups.
In USR_MANAGER only the user can search groups, the USER_MANAGER, which
is used to bind in this mode, can not.

A third server instance is created because I did fear side effects should
the tests be run in parallel, had I tried to configure the access
restriction in Before.