mgcp_test: fix log of conn_id presence

Flip logic to accurately log whether an 'I:' is included, instead of logging
the opposite.

Note that it isn't possible to log the actual conn ID, because they are random
and differ in every test run, which would collide with the fixed expected
output file mgcp_test.ok.

Change-Id: Idcd731b9daf618b97d8f7e6a776266071cd29e08
diff --git a/tests/mgcp/mgcp_test.c b/tests/mgcp/mgcp_test.c
index f8f701e..228b2f9 100644
--- a/tests/mgcp/mgcp_test.c
+++ b/tests/mgcp/mgcp_test.c
@@ -737,7 +737,7 @@
 		if (msg) {
 			rc = get_conn_id_from_response(msg->data, last_conn_id,
 						       sizeof(last_conn_id));
-			if (rc)
+			if (rc == 0)
 				printf("(response contains a connection id)\n");
 			else
 				printf("(response does not contain a connection id)\n");