Changes

ggsn: testenv: run SUT on bridge instead of lo

Replace IPs in testenv.sh so the SUT runs on 172.18.3.2 (testenv0
bridge) instead of 127.0.0.2 (lo). Later on when we can optionally use
QEMU with osmo-ggsn to test kernel GTP-U. It will then run on this IP as
well. So with this change we can use the same IP for both the QEMU and
non-QEMU case.

Change-Id: Ib768603b0050fe62baff39cab982a850eb91f49a