gprs: ping.py: Disable ipv6 ctx activate test

osmo-ggsn is failing to create tun ipv6 device in Prod main unit.
Afterwards, as the iface is not created, it cannot find its link-local
ip and it doesn't configure the ipv6 pool. Later on, when an ipv4v6 ctx
is requested, it fails because apn doesn't support ipv6 (because the
pool is not created).

Related: OS#2746

Change-Id: I018c525a8a3d108233740ee1376b2671fefbbb59
1 file changed