sgsn: add Iu Attach Request

MS -> SGSN: Attach Request IMSI
MS <- SGSN: Identity Request IMEI
MS -> SGSN: Identity Response IMEI
MS <- SGSN: Auth Request
MS -> SGSN: Auth Response
MS <- SGSN: Attach Accept
MS -> SGSN: Attach Complete

Change-Id: I66069e31c30d33934ad57cc2b8794f56ffd5c7d6
diff --git a/sgsn/SGSN_Tests.ttcn b/sgsn/SGSN_Tests.ttcn
index 4a47723..adbcf19 100644
--- a/sgsn/SGSN_Tests.ttcn
+++ b/sgsn/SGSN_Tests.ttcn
@@ -11,6 +11,8 @@
  * SPDX-License-Identifier: GPL-2.0-or-later
  */
 
+friend module SGSN_Tests_Iu;
+
 import from General_Types all;
 import from Osmocom_Types all;
 import from Native_Functions all;