NSS/TESTS: Improve non-fqnames tests
    
    With the changes that are about to happen we have to have the subdomain's
    fqnames flag set by the time we populate the cr_domains list (as it
    actually occurs with the real code), as this list may set its own fqnames
    flag based on the subdomain's fqnames flag.
    
    Currently the flag is set to false only when running the tests itself so
    the cr_domains list doesn't get populate properly (although it still
    works with the current code).
    
    For the changes that are comming, let's introduce a new setup function
    that ensures that the subdomain's fqnames flag is set up in the right
    time.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3001
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>