SYSDB/TESTS: Add tests for the domain's resolution order methods
    
    Introduce a new and small set of tests for these new helper methods that
    are going to be used in different parts of the code in the follow-up
    patches.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3001
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>