ggsn: TC_pdp(4)6_clients_interact: Expect UEs can interact using global IPv6 address
The link-local IPv6 address is only meant to be used privately between the UE and the GGSN, but the UEs should be in general able to reach each other using their global IPv6 address (unless some operator policy explicitly forbids it).
Since open5gs.git 6cc627c4855703a17c92352660078325c6ce63cc, open5gs-upfd supports proper routing of IPv6 packets between UEs.
osmo-ggsn apparently doesn't implement that internal routing logic yet, so these tests will start failing now, but it's actually legit to have it not passing the test anymore.
