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>