Refactors tailnet integration test and adds UDP echo tests with different MTU related to #15523

I still haven't gotten to the bottom of what's causing the issue (the added test case I expected to fail actually succeeds), but these integration test improvements are generally useful.

also:

• consolidates networking setup with easy and hard NAT
• consolidates client setup
• makes Client2 act like an agent at the tailnet layer, so it will send ReadyForHandshake and speed up the tunnel establishment
• adds support for logging tunneled packets
• adds support for dumping outer (underlay) IP traffic
• adds support for adjusting veth MTU
• adds support for IPv6 in the outer (underlay) network topology