TESTS: Add a basic SSH responder test
    
    Adds a basic test that makes sure that a list of SSH public keys can be
    retrieved. This is to make sure we don't break the SSH integration later
    on.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3747
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>