selftests: ublk: add --foreground command line

Add --foreground command for helping to debug.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250303124324.3563605-4-ming.lei@redhat.com


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