sgsn_test: Adapt test case to now-existing InsertSubscriberData

We recently implementd InsertSubscriberData in the SGSN, adapt the test
to reflect that.
diff --git a/openbsc/tests/sgsn/sgsn_test.c b/openbsc/tests/sgsn/sgsn_test.c
index 5cffff2..afff30f 100644
--- a/openbsc/tests/sgsn/sgsn_test.c
+++ b/openbsc/tests/sgsn/sgsn_test.c
@@ -610,8 +610,8 @@
 	/* Inject InsertSubscrData GSUP message */
 	last_updated_subscr = NULL;
 	rc = rx_gsup_message(insert_data_req, sizeof(insert_data_req));
-	OSMO_ASSERT(rc == -GMM_CAUSE_MSGT_NOTEXIST_NOTIMPL);
-	OSMO_ASSERT(last_updated_subscr == NULL);
+	OSMO_ASSERT(rc = -ENOTSUP);	/* not connected */
+	OSMO_ASSERT(last_updated_subscr == s1);
 
 	/* Inject DeleteSubscrData GSUP message */
 	last_updated_subscr = NULL;