Since osmo-ggsn.git Ia15c1cfd201d7c43e9a1d6ceb6725ddf392d2c65 osmo-ggsn
supports configuration of X3 timer, which should be set to (T3-RESPONSE
* N3-REQUESTS) configured at the peer.
The default values are 5 and 3 respectively, hence X3 is by default
5*3=15 seconds.
Let's use that default value for now.
Once new osmo-ggsn version is released, we can speed up test duration by
decreasing the values of the module parameters introduced in this commit
and configure VTY gtp timers at osmo-ggsn accordingly.
Related: OS#5485
Change-Id: I02c0982674b43317a5fc8f341c03eeeb1efee77f