iotests/264: add mirror-cancel test-case

Check that cancel doesn't wait for 10s of nbd reconnect timeout.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210205163720.887197-9-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>