selftests: ublk: fix starting ublk device

Firstly ublk char device node may not be created by udev yet, so wait
a while until it can be opened or timeout.

Secondly delete created ublk device in case of start failure, otherwise
the device becomes zombie.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250321135324.259677-1-ming.lei@redhat.com


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