Fix bug in ldapBackend with LDAP_AUTH_GROUP

If LDAP_AUTH_GROUP was not defined in settings, ldapBackend would
raise an Exception making authentication impossible. Fix it using the
canonical way of checking for dictionary key existence
DRY optimization in ldapBackend

Just some simple code deduplication
Add some extra LDAP auth unit tests

A bad login
A not correctly created user but correct password
A malformed user