blktests: add Ming Lei's scsi-stress-remove

blktests: add Ming Lei's scsi-stress-remove

This test exposed a race condiiton when shutting down a request_queue
with active IO against it and blkg association for the IOs [1]. The
issue ended up being that while the request_queue will just start
failing requests, blkg destruction sets the q->root_blkg to %NULL. This
caused a NPE. This was fixed in [2].

So to help prevent this from happening again, integrate Ming's test into
blktests so that it can more easily be ran. Here I've ported it to fit
better into the blktests framework.

[1] https://lore.kernel.org/lkml/20181205171039.73066-1-dennis@kernel.org/
[2] https://lore.kernel.org/lkml/20181211230308.66276-1-dennis@kernel.org/

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Cc: Ming Lei <ming.lei@redhat.com>