vmtest: add VM-based integration test framework

Add tstest/natlab/vmtest, a high-level framework for running multi-VM
integration tests with mixed OS types (gokrazy + Ubuntu/Debian cloud
images) connected via natlab's vnet virtual network.

The vmtest package provides:
  - Env type that orchestrates vnet, QEMU processes, and agent connections
  - OS image support (Gokrazy, Ubuntu2404, Debian12) with download/cache
  - QEMU launch per OS type (microvm for gokrazy, q35+KVM for cloud)
  - Cloud-init seed ISO generation with network-config for multi-NIC
  - Cross-compilation of test binaries for cloud VMs
  - Debug SSH NIC on cloud VMs for interactive debugging
  - Test helpers: ApproveRoutes, HTTPGet, TailscalePing, DumpStatus,
    WaitForPeerRoute, SSHExec

TTA enhancements (cmd/tta):
  - Parameterize /up (accept-routes, advertise-routes, snat-subnet-routes)
  - Add /set, /start-webserver, /http-get endpoints
  - /http-get uses local.Client.UserDial for Tailscale-routed requests
  - Fix /ping for non-gokrazy systems

TestSubnetRouter exercises a 3-VM subnet router scenario:
  client (gokrazy) → subnet-router (Ubuntu, dual-NIC) → backend (gokrazy)
  Verifies HTTP access to the backend webserver through the Tailscale
  subnet route. Passes in ~30 seconds.

Updates tailscale/tailscale#13038

Change-Id: I165b64af241d37f5f5870e796a52502fc56146fa
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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