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Verifies that site-to-site Tailscale subnet routing with --snat-subnet-routes=false preserves the original source IP end-to-end. Topology: two sites, each with a Linux subnet router on a NATted WAN plus an internal LAN, and a non-Tailscale backend on each LAN. Backends are given static routes pointing to their local subnet router for the remote site's prefix; an HTTP GET from backend-a to backend-b over Tailscale returns a body containing backend-a's LAN IP. Adds the supporting vmtest.SNATSubnetRoutes NodeOption and plumbs snat-subnet-routes through TTA's /up handler. The webserver started by vmtest.WebServer now also echoes the remote IP, for the preservation assertion. Adds a /add-route TTA endpoint (Linux-only for now) and a vmtest Env.AddRoute helper so the test can install the backend static routes through TTA rather than needing a host SSH key and debug NIC. ensureGokrazy now always rebuilds the natlab qcow2 (once per test process, via sync.Once) so the test picks up the new TTA and webserver behavior. This is pulled out of a larger pending change that adds FreeBSD site-to-site subnet routing support; figured we should have at least the Linux test covering what works today. Updates #5573 Change-Id: I881c55b0f118ac9094546b5fbe68dddf179bb042 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>