selftests: ublk: cleanup backfile automatically

Use global array of $UBLK_BACKFILES for storing all backfile name, then
clean them automatically.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-4-ming.lei@redhat.com


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