test GGSN support for optional GTP-U sequence numbers

Add VTY functionality to GGSN tests, and use the VTY to enable/disable
GTP-U Tx sequence numbers in the running osmo-ggsn.

The GTPU packet template now makes sequence numbers optional.
A template created with its sequence number set to 'omit' will result in
a packet without a sequence number, i.e. the 'sequence number present' bit
in the packet header is cleared, and the sequence number field is omitted
from the encoded GTPU T-PDU packet.

Re-use the existing TC_pdp4_clients_interact() test for testing the
behaviour of osmo-ggsn. This test is now run twice, once with and
once without GTP-U Tx sequence numbers. Verify that packets relayed by
osmo-ggsn match its "g-pdu tx-sequence-numbers" configuration setting.

Change-Id: I1dc299407c61b1c865035add44067b8ab89001b3
Related: OS#2519
diff --git a/ggsn_tests/gen_links.sh b/ggsn_tests/gen_links.sh
index 6ef52e1..4e012ae 100755
--- a/ggsn_tests/gen_links.sh
+++ b/ggsn_tests/gen_links.sh
@@ -40,9 +40,14 @@
 FILES="GTPC_EncDec.cc  GTPC_Types.ttcn  GTPU_EncDec.cc  GTPU_Types.ttcn"
 gen_links $DIR $FILES
 
+DIR=$BASEDIR/titan.TestPorts.TELNETasp/src
+FILES="TELNETasp_PT.cc  TELNETasp_PT.hh  TELNETasp_PortType.ttcn"
+gen_links $DIR $FILES
+
 DIR=../library
 FILES="General_Types.ttcn GSM_Types.ttcn Osmocom_Types.ttcn Native_Functions.ttcn Native_FunctionDefs.cc IPCP_Types.ttcn "
 FILES+="GTP_CodecPort.ttcn GTP_CodecPort_CtrlFunct.ttcn GTP_CodecPort_CtrlFunctDef.cc GTP_Templates.ttcn "
+FILES+="Osmocom_VTY_Functions.ttcn "
 gen_links $DIR $FILES
 
 ignore_pp_results