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>

tstest/natlab/vmtest/vmtest_test.go

@@ -13,6 +13,15 @@ import (
 )
 
 func TestSubnetRouter(t *testing.T) {
+	testSubnetRouterForOS(t, vmtest.Ubuntu2404)
+}
+
+func TestSubnetRouterFreeBSD(t *testing.T) {
+	testSubnetRouterForOS(t, vmtest.FreeBSD150)
+}
+
+func testSubnetRouterForOS(t testing.TB, srOS vmtest.OSImage) {
+	t.Helper()
 	env := vmtest.New(t)
 
 	clientNet := env.AddNetwork("2.1.1.1", "192.168.1.1/24", "2000:1::1/64", vnet.EasyNAT)
@@ -21,7 +30,7 @@ func TestSubnetRouter(t *testing.T) {
 	client := env.AddNode("client", clientNet,
 		vmtest.OS(vmtest.Gokrazy))
 	sr := env.AddNode("subnet-router", clientNet, internalNet,
-		vmtest.OS(vmtest.Ubuntu2404),
+		vmtest.OS(srOS),
 		vmtest.AdvertiseRoutes("10.0.0.0/24"))
 	backend := env.AddNode("backend", internalNet,
 		vmtest.OS(vmtest.Gokrazy),