add tests for AutoNAT

We should have a test that checks that nodes can reliably determine if they are behind a NAT. This will most likely require us to implement some kind of NAT (equivalent?), maybe using `iptables`.

Main challenge:
* Implementing the NAT.
* AutoNAT requires a few interactions with different nodes, so we’d have to boot up a small network of helper nodes.

The issue names no files or existing tests. Start by reviewing the AutoNAT behavior and the repository's current test approach, then investigate whether iptables can provide the required NAT and how helper nodes can form a small network. Done means a repeatable test reliably distinguishes nodes behind NAT.

Written by the indexing model from the issue text.