tstest/natlab/vmtest: kill QEMU when test process dies (#19676)

Re-exec the test binary as a thin wrapper that holds a pipe inherited
from the test. When the test goes away (any reason, including SIGKILL,
panic, or OOM), the kernel closes the pipe write end; the wrapper sees
EOF and SIGKILLs itself, taking QEMU and its children with it.

Updates #13038

Change-Id: Ib2151098193551396c1d7bb51b07da3bd6b2cfb4

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

tstest/natlab/vmtest/qemu_wrapper_windows.go

@@ -0,0 +1,20 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package vmtest
+
+import (
+	"os"
+	"os/exec"
+)
+
+// Stubs for Windows: no parent-death watcher, no process-group kill.
+// The test still launches QEMU; cleanup just kills the single process.
+
+func killWithParent(cmd *exec.Cmd) (*os.File, error) {
+	return os.Open(os.DevNull)
+}
+
+func killProcessTree(cmd *exec.Cmd) error {
+	return cmd.Process.Kill()
+}