tstest/natlab: add TestSubnetRouterFreeBSD with FreeBSD cloud image support

As a warm-up to making natlab support multiple operating systems,
start with an easy one (in that it's also Unixy and open source like
Linux) and add FreeBSD 15.0 as a VM OS option for the vmtest
integration test framework, and add TestSubnetRouterFreeBSD which
tests subnet routing through a FreeBSD VM (Gokrazy → FreeBSD →
Gokrazy).

Key changes:
- Add FreeBSD150 OSImage using the official FreeBSD 15.0
  BASIC-CLOUDINIT cloud image (xz-compressed qcow2)
- Add GOOS()/IsFreeBSD() methods to OSImage for cross-compilation
  and OS-specific behavior
- Handle xz-compressed image downloads in ensureImage
- Refactor compileBinaries into compileBinariesForOS to support
  multiple GOOS targets (linux, freebsd), with binaries registered
  at <goos>/<name> paths on the file server VIP
- Add FreeBSD-specific cloud-init (nuageinit) user-data generation:
  string-form runcmd (nuageinit doesn't support YAML arrays),
  fetch(1) instead of curl, FreeBSD sysctl names for IP forwarding,
  mkdir /usr/local/bin, PATH setup for tta
- Skip network-config in cidata ISO for FreeBSD (DHCP via rc.conf)

Updates tailscale/tailscale#13038

Change-Id: Ibeb4f7d02659d5cd8e3a7c3a66ee7b1a92a0110d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

go.mod

@@ -471,7 +471,7 @@ require (
 	github.com/tomarrell/wrapcheck/v2 v2.8.3 // indirect
 	github.com/tommy-muehle/go-mnd/v2 v2.5.1 // indirect
 	github.com/u-root/uio v0.0.0-20240224005618-d2acac8f3701 // indirect
-	github.com/ulikunitz/xz v0.5.15 // indirect
+	github.com/ulikunitz/xz v0.5.15
 	github.com/ultraware/funlen v0.1.0 // indirect
 	github.com/ultraware/whitespace v0.1.0 // indirect
 	github.com/uudashr/gocognit v1.1.2 // indirect