tests: add a regression test for raid456 deadlock

tests: add a regression test for raid456 deadlock

The deadlock is described in [1], as the last patch described, it's
fixed first by [2], however this fix will be reverted and the deadlock
is supposed to be fixed by [3].

[1] https://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t
[2] https://lore.kernel.org/linux-raid/20220621031129.24778-1-guoqing.jiang@linux.dev/
[3] https://lore.kernel.org/linux-raid/20230322064122.2384589-5-yukuai1@huaweicloud.com/

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>