PGW_Tests: fix f_create_session(): add missing Serving Network IE

open5gs-smfd version 2.4.2 requires this IE to be present:

[smf] INFO: [Added] Number of SMF-UEs is now 1 (../src/smf/context.c:820)
[smf] INFO: [Added] Number of SMF-Sessions is now 1 (../src/smf/context.c:2610)
[smf] ERROR: No Serving Network (../src/smf/s5c-handler.c:101)
[smf] ERROR: No Data in Serving Network (../src/smf/s5c-handler.c:105)
[smf] ERROR: Invalid Len[0] in Serving Network (../src/smf/s5c-handler.c:110)

Change-Id: Idf6d147cc4a37996ee4ae6de18158b0e140d9db6
Related: SYS#5602
diff --git a/pgw/PGW_Tests.ttcn b/pgw/PGW_Tests.ttcn
index 81e3552..f026e58 100644
--- a/pgw/PGW_Tests.ttcn
+++ b/pgw/PGW_Tests.ttcn
@@ -305,6 +305,7 @@
 	var template (value) TAI tai := { '0'H, '0'H, '1'H, 'F'H, '0'H, '1'H, '0001'O };
 	var template (value) ECGI ecgi := { '0'H, '0'H, '1'H, 'F'H, '0'H, '1'H, '0'H, 23 };
 	g2c.gtpcv2_pdu.createSessionRequest.userLocationInfo := ts_GTP2C_UserLocInfo(tai := tai, ecgi := ecgi);
+	g2c.gtpcv2_pdu.createSessionRequest.servingNetwork := ts_GTP2C_ServingNetwork('001'H, '01F'H);
 
 	GTP2.send(g2c);
 	if (DIAMETER_PROC.checkstate("Connected")) {