Changes

ggsn: Split test and relax expectations testing gtpu access over ipv6 ll-addr

Testing gtpu traffic to some potentially external IP address using the
link-local address has no clear usecase and it also depends on the
network setup and the implementation of the GGSN.
Hence, remove it from the regular test and move it to another test which
has a relaxed validation accepting different resolutions.
This allows knowing/testing the code path in the GGSN, while accepting
different outcomes.

Related: OS#6382
Change-Id: Icfae181ba1bb0dfb8a1a15dfe3a1d542ce69d88a