selftests/nolibc: exit with poweroff on success when getpid() == 1

The idea is to ease automated testing under qemu. If the test succeeds
while running as PID 1, indicating the system was booted with init=/test,
let's just power off so that qemu can exit with a successful code. In
other situations it will exit and provoke a panic, which may be caught
for example with CONFIG_PVPANIC.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>