RESPONDER: Use fqnames as output when needed
    
    As some regressions have been caused by not handling properly naming
    conflicts when using shortnames, last explicitly use fully qualified
    names as output in the following situations:
    - domain resolution order is set;
    - a trusted domain has been using `use_fully_qualified_name = false`
    
    In both cases we want to ensure that even handling shortnames as input,
    the output will always be fully qualified.
    
    As part of this patch, our tests ended up being modified to reflect the
    changes done. In other words, the tests related to shortnames now return
    expect as return a fully qualified name for trusted domains.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3403
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>