dyndns_tests: Fix unit test with missing features in nsupdate
    
    We return different string in the function nsupdate_msg_add_realm_cmd
    if realm command is not supported by nsupdate.
    However cmocka based unit test did not expect such string and failed.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 44996f2ea96ce75bf76f5155a6bdecf295593d97)