selftests: ublk: add two stress tests for zero copy feature

Add stress_03 & stress_04 for covering zero copy feature.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-7-ming.lei@redhat.com


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