dyndns-tests: Fix false positive failures
    
    The child process finished faster then it has handled by parent
    and therefore it timed out. It's the similar solution as in
    b3074dca3acebd91437ef13d3329d6d65d655215
    
    [ RUN      ] dyndns_test_error
    (Fri Jul 29 16:12:00:621444 2016) [sssd] [nsupdate_child_timeout] (0x0020):
      Timeout reached for dynamic DNS update
    Could not run the test - check test fixtures
    [  ERROR   ] dyndns_test_error
    
    Reviewed-by: Petr Čech <pcech@redhat.com>
    (cherry picked from commit bad17a253bf979f23b2294c3720300d1053744b1)