ggsn_tests: Add test to validate if different pdp ctx can talk each other

It seems due to the current network configuration, pdp v4 ctx can talk
each other while pdp v6 ctx cannot.

Change-Id: I67c04b056cc5c092d357abbb084b7665f59eaf3a
diff --git a/ggsn_tests/GGSN_Tests.cfg b/ggsn_tests/GGSN_Tests.cfg
index bf82057..67fcbd4 100644
--- a/ggsn_tests/GGSN_Tests.cfg
+++ b/ggsn_tests/GGSN_Tests.cfg
@@ -27,10 +27,12 @@
 GGSN_Tests.TC_pdp4_act_deact_ipcp
 GGSN_Tests.TC_pdp4_act_deact_pcodns
 GGSN_Tests.TC_pdp4_act_deact_gtpu_access
+GGSN_Tests.TC_pdp6_clients_interact
 
 GGSN_Tests.TC_pdp6_act_deact
 GGSN_Tests.TC_pdp6_act_deact_pcodns
 GGSN_Tests.TC_pdp6_act_deact_icmp6
 GGSN_Tests.TC_pdp6_act_deact_gtpu_access
+GGSN_Tests.TC_pdp4_clients_interact
 
 GGSN_Tests.TC_echo_req_resp