GGSN_Tests: Make sure f_init() body is traversed only once

We don't want the sequence numbers to be random-reinitialized
on every test case, but only once.
diff --git a/ggsn_tests/GGSN_Tests.ttcn b/ggsn_tests/GGSN_Tests.ttcn
index 9408121..2066e0d 100644
--- a/ggsn_tests/GGSN_Tests.ttcn
+++ b/ggsn_tests/GGSN_Tests.ttcn
@@ -40,6 +40,8 @@
 		port GTPC_PT GTPC;
 		port GTPU_PT GTPU;
 
+		var boolean g_initialized := false;
+
 		var OCT1 g_restart_ctr := '01'O;
 		/* FIXME: unify with g_bind_ip + parse from config file */
 		var OCT4 g_sgsn_ip_c := '7f172a01'O;
@@ -56,6 +58,11 @@
 	}
 
 	function f_init() runs on GT_CT {
+		if (g_initialized == true) {
+			return;
+		}
+		g_initialized := true;
+
 		var Result res;
 		map(self:GTPC, system:GTPC);
 		res := GTP_CodecPort_CtrlFunct.f_IPL4_listen(GTPC, g_bind_ip_c, GTP1C_PORT, {udp:={}});