ttcn3-ggsn: Speed up tests by lowering retransmission T3,N3 timers
Every test needs to wait for T3*N3 seconds before exiting in order to make sure the queues waiting for duplicates are cleared, otherwise state from previous state may affect next tests. Since 1.10.0, osmo-ggsn supports having those timers changed through VTY. Let's set them to a lower value than default specified by spec, in order to speed up each test 15 seconds -> 2 seconds.
Only ttcn3-ggsn (against osmo-ggsn) and not ttcn3-ggsn-ogs (against open5gs-smfd) is updated, since the later doesn't support changing the default timer values using the config files yet.
