[2416] Wait for AUTH_LOAD_ZONE in lettuce test

For the SOA=1239 case, lettuce (instead of waiting for a new
AUTH_LOAD_ZONE message) waits for the last output AUTH_LOAD_ZONE
message because the last SOA=1238 case generated a AUTH_LOAD_ZONE that
was not skipped past in RunningProcess._wait_for_output_str().  So it
immediately matches the last AUTH_LOAD_ZONE and continues right away
which causes it to query Auth prematurely.

tests/lettuce/features/ddns_system.feature

@@ -53,6 +53,7 @@ Feature: DDNS System
 
         # Test 8
         When I use DDNS to set the SOA serial to 1238
+        And wait for new bind10 stderr message AUTH_LOAD_ZONE
         The DDNS response should be SUCCESS
         And the SOA serial for example.org should be 1238