migrate ttcn3-msc-test from 172.18.1.0/24 to 172.18.20.0/24
When introducing IPv6 support, we map the third digit of the IPv4 address (X) to the 6th byte of an IPv6 prefix "fd02:db8:X::/64"
However, the docker daemon seems to use "fd02:db8:1::/64" internally for its default network, so creating a docker network with the same IP address is failing.
Let's move the MSC test suite to another sub-net (1->20) to avoid related problems.
