tests: remove LOCAL_SYSDB_FILE reference from test_sysdb_domain_resolution_order_
    
    Let's take advantage of the files provider and start to get rid of the
    local provider references in our code.
    
    Let's just replace "LOCAL_SYSDB_FILE" for NULL as This test suite
    doesn't create local sysdb file.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3304
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>