ttcn3-ggsn-tests-ogs: Set correct DNS address to get ping working
The DNS address provided is used by the test to ping. In osmo-ggsn, we configured the DNS to be the shared address between GTPC and GTPU. In open5gs, the addresses are different. Hence, let's pass the GTPU address instead of the GTPC address to make the setup easier (no need to have proper routing working between UPFD and SMFD containers).
