selftests: ublk: run stress tests in parallel

Run stress tests in parallel, meantime add shell local function to
simplify the two stress tests.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-6-ming.lei@redhat.com


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