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/regen_makefile.sh b/ggsn_tests/regen_makefile.sh
index 2fc74f7..6db1a3d 100755
--- a/ggsn_tests/regen_makefile.sh
+++ b/ggsn_tests/regen_makefile.sh
@@ -1,5 +1,5 @@
 #!/bin/sh
 
-FILES="*.ttcn IPL4asp_PT.cc  IPL4asp_discovery.cc  TCCConversion.cc  TCCInterface.cc GTPC_EncDec.cc GTPU_EncDec.cc GTP_CodecPort_CtrlFunctDef.cc ICMPv6_EncDec.cc IP_EncDec.cc Native_FunctionDefs.cc UDP_EncDec.cc ICMP_EncDec.cc"
+FILES="*.ttcn IPL4asp_PT.cc  IPL4asp_discovery.cc  TCCConversion.cc  TCCInterface.cc GTPC_EncDec.cc GTPU_EncDec.cc GTP_CodecPort_CtrlFunctDef.cc ICMPv6_EncDec.cc IP_EncDec.cc Native_FunctionDefs.cc UDP_EncDec.cc ICMP_EncDec.cc TELNETasp_PT.cc"
 
 ../regen-makefile.sh GGSN_Tests.ttcn $FILES