test-bdrv-drain: don't use BlockJob.blk

We are going to drop BlockJob.blk in further commit. For tests it's
enough to simply pass bs pointer.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Nikita Lapshin <nikita.lapshin@virtuozzo.com>