ggsn_tests: Add test case to validate packets forwarded vs dropped

New dependency is required: titan.ProtocolModules.ICMP
It tests that ICMP echo packets can be sent successfully (reply is
received or otherwise dest unreachable if routing is not set up
correctly during the test). It also tests some cases in which osmo-ggsn
is required to drop the packets (eg. unknown src ip unrelated to pdp
ctx). It also checks that IPv6 packets are dropped in IPv4 pdp ctx and
viceversa It also checks that IPv6 packets are dropped in IPv4 pdp ctx
and vice versa.

Change-Id: Ib9c6043a6cd3b6622782ec7e7fcd2815101755ba
4 files changed