test/poll-cancel: improve test case
test/poll-cancel: improve test case

- Add a timeout, if we don't finish in one second we are definitely
  hung because the poll remove command didn't work.
- Don't assume completion ordering

Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing