stp: Fix sporadic failure in TC_ipa_to_m3ua_ni

It was spotted that from time to time the test set NI international
instead of national. This is presumable due to a race condition between
VTY and messages being received and forwarded.
Let's delay resetting the VTY config after the message is recieved, to
avoid the race conditions between them.

Change-Id: I3ef08447d2e36b6948d3db4ec9d1459beebc9384

stp/STP_Tests.ttcn

@@ -144,14 +144,13 @@ testcase TC_ipa_to_m3ua_ni() runs on IPA_M3UA_CT {
 	var octetstring sccp_enc := enc_PDU_SCCP(sccp);
 	f_vty_config2(VTY, {"cs7 instance 0"}, "network-indicator national");
 	f_IPA_send(3, sccp_enc);
-	f_vty_config2(VTY, {"cs7 instance 0"}, "network-indicator international");
 
 	/* expect to receive it via M3UA */
 	var template (present) M3UA_Protocol_Data rx_pd;
 	rx_pd := tr_M3UA_protocol_data(pc_sender, pc_receiver, c_M3UA_SI_SCCP, c_M3UA_NI_NATIONAL,
 					'00'O, '00'O, sccp_enc);
 	f_M3UA_exp(0, tr_M3UA_DATA(rctx_receiver, rx_pd));
-
+	f_vty_config2(VTY, {"cs7 instance 0"}, "network-indicator international");
 	f_clear_m3ua();
 }