selftests: ublk: add variable for user to not show test result

Some user decides test result by exit code only, and wouldn't like to be
bothered by the test result.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250320013743.4167489-4-ming.lei@redhat.com


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