[tests] give PMTUd more time to redetect MTU

Ideal fix would be to use PMTUd callback, but that requires a lot of
extra test infrastructure. For now just workaround the problem.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>

libknet/tests/api_knet_handle_pmtud_set.c

@@ -196,7 +196,7 @@ static void test(void)
 	/*
 	 * wait for PMTUd to pick up the change
 	 */
-	sleep(1);
+	sleep(2);
 	flush_logs(logfds[0], stdout);
 
 	if (knet_h->data_mtu != data_mtu) {