Fixes#15523
Uses latest https://github.com/coder/tailscale which includes https://github.com/coder/tailscale/pull/85 to stop selecting paths with small MTU for direct connections.
Also updates the tailnet integration test to reproduce the issue. The previous version had the 2 peers connected by a single veth, but this allows the OS to fragment the packet. In the new version, the 2 peers (and server) are all connected by a central router. The link between peer 1 and the router has an adjustable MTU. IPv6 does not allow packets to be fragmented by intermediate routers, so sending a too-large packet in this scenario forces the router to drop packets and reproduce the issue (without the tailscale changes).
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
- `DERPForceWebSockets`: Test that DERP over WebSocket (as well as DERPForceWebSockets works). This does not test the actual DERP failure detection code and automatic fallback.
- `DERPFallbackWebSockets`: Test that falling back to DERP over WebSocket works.
Also:
- Rearranges some test code and refactors `TestTopology.StartServer` to be `TestTopology.ServerOptions` and take a struct instead of a function
Closes#13045
Spike Curtis