Add a new framework for testing out how particular packets would be
handled by a given set of iptables rules. (eg, "assert that a packet
from 10.180.0.2 to 172.30.0.41:80 gets NATted to 10.180.0.1:80 without
being masqueraded"). Add tests using this to all of the existing unit
tests.
This makes it easier to tell whether a given code change has any
effect on behavior, without having to carefully examine the diffs to
the generated iptables rules.