testing: Add a connmark plugin test

testing: Add a connmark plugin test

In this test two hosts establish a transport mode connection from behind
moon. sun uses the connmark plugin to distinguish the flows.

This is an example that shows how one can terminate L2TP/IPsec connections
from two hosts behind the same NAT. For simplification of the test, we use
an SSH connection instead, but this works for any connection initiated flow
that conntrack can track.