selftests: ublk: add stress test for covering IO vs. killing ublk server

selftests: ublk: add stress test for covering IO vs. killing ublk server

Add stress_test_01 for running IO vs. killing ublk server, so io_uring exit &
cancel code path can be covered, same with ublk's cancel code path.

Especially IO buffer lifetime is one big thing for ublk zero copy, the added
test can verify if this area works as expected.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250303124324.3563605-11-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>