TT#36275 Increase wait time in invite_callforward scenario

The errors is not due to re-transmission, but packet ordering.
It seems that, in high loaded systems, kamailio proxy takes too
much time to process some packages causing an unexpected order.

Increasing the wait time from 500ms to 600ms give 100ms more to the
proxy to process the packet and send it out in the expected order.

Change-Id: Idccd6c7b95c428cbee519bb6cf779c6a8dfc3e82

scenarios/invite_callforward/sipp_scenario_responder03.xml

@@ -20,7 +20,7 @@
     ]]>
   </send>
 
-  <pause milliseconds="500"/>
+  <pause milliseconds="600"/>
 
   <send>
     <![CDATA[