tests/block/031: Add a test for sharing a tag set across hardware queues

tests/block/031: Add a test for sharing a tag set across hardware queues

Support for sharing a tag set across hardware queues has been added
recently to the Linux kernel. See also the BLK_MQ_F_TAG_HCTX_SHARED flag,
Linux kernel commit 32bc15afed04 ("blk-mq: Facilitate a shared sbitmap per
tagset"; v5.10) and commit 0905053bdb5b ("null_blk: Support shared tag
bitmap"; v5.10). Add a test that triggers the shared tag set code in the
block layer core.

Cc: John Garry <john.garry@huawei.com>
Cc: Don Brace<don.brace@microsemi.com>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>