test/waitid: be nice and wait for unrepeaped forks

There are some test cases in waitid that forks off multiple children
without waiting for them to exit. This can create zombies in some
environments. Be nice and wait for them manually, if we deliberately
don't reap them via the io_uring WAITIID command.

Signed-off-by: Jens Axboe <axboe@kernel.dk>