add tests for drain io with multishot reqs

Add a simple test for drain io with multishot reqs. A generic one as
well, which randomly generates sqes for testing. The later will cover
most cases.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
[axboe: silence read not-checked error]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
3 files changed