tests: Add tests for idoverride object integrity
    
    As far as IPA objects are concerned, ID overrides are supposed
    to be removed when the respective user/group is removed.
    
    Adds a couple of tests to ensure this behaviour is covered.
    
    https://fedorahosted.org/freeipa/ticket/5322
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>