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>