Changes

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.

Related: OS#5485
Change-Id: Ica346bd791a67b72fad597702d750383e8ac5895