stp: Don't expect a fixed SLS in IPA -> M3UA case

We are about to introduce a feature for SLS "randomization" in osmo-stp,
so let's modifiy our STP tests to permit any SLS in the M3UA messages
after IPA -> M3UA conversion.

Change-Id: Ib70322b80a72e3e7dc3155bca9e67b03efb413e0
Related: SYS#6543
diff --git a/stp/STP_Tests.ttcn b/stp/STP_Tests.ttcn
index 48214be..e2e5336 100644
--- a/stp/STP_Tests.ttcn
+++ b/stp/STP_Tests.ttcn
@@ -114,7 +114,7 @@
 
 	/* 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, '00'O, '00'O, '00'O, sccp_enc);
+	rx_pd := tr_M3UA_protocol_data(pc_sender, pc_receiver, c_M3UA_SI_SCCP, '00'O, '00'O, ?, sccp_enc);
 	f_M3UA_exp(0, tr_M3UA_DATA(rctx_receiver, rx_pd));
 
 	f_clear_m3ua();
@@ -148,7 +148,7 @@
 	/* 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);
+					'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();
@@ -185,7 +185,7 @@
 
 	/* 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, '00'O, '00'O, '00'O,
+	rx_pd := tr_M3UA_protocol_data(pc_sender, pc_receiver, c_M3UA_SI_SCCP, '00'O, '00'O, ?,
 					enc_PDU_SCCP(sccp_exp));
 	f_M3UA_exp(0, tr_M3UA_DATA(rctx_receiver, rx_pd));