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/library/GTPv2_Templates.ttcn b/library/GTPv2_Templates.ttcn
index 3d69017..644728e 100644
--- a/library/GTPv2_Templates.ttcn
+++ b/library/GTPv2_Templates.ttcn
@@ -1035,6 +1035,25 @@
 		privateExtension := *
 	}});
 
+/* 8.18 */
+template (value) ServingNetwork
+ts_GTP2C_ServingNetwork(template (value) hexstring mcc,
+			template (value) hexstring mnc,
+			template (value) BIT4 instance := '0000'B) :=
+{
+	elementIdentifier := '53'O,
+	lengthIndicator := 0, /* overwritten */
+	instance := instance,
+	spare := '0000'B,
+	mccDigit1 := mcc[0],
+	mccDigit2 := mcc[1],
+	mccDigit3 := mcc[2],
+	mncDigit3 := mnc[2], /* 'F'H for 2 digit MNC */
+	mncDigit1 := mnc[0],
+	mncDigit2 := mnc[1],
+	additionalOctets := omit
+}
+
 
 
 }