tstest/natlab/vmtest,cmd/tta: add TestSiteToSite

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>

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