tests-extra: reduce sleep (2 sec) interval between lsof calls

The previous value (4 sec) extended ixfr/from_differences execution
time by 40 minutes!

tests-extra/tools/dnstest/server.py

@@ -184,7 +184,7 @@ class Server(object):
             if len(pids) == 1 and str(self.proc.pid) in pids:
                 return True
 
-            time.sleep(4)
+            time.sleep(2)
 
         return False