iotest 151: add test-case that shows active mirror dead-lock

iotest 151: add test-case that shows active mirror dead-lock

There is a dead-lock in active mirror: when we have parallel
intersecting requests (note that non intersecting requests may be
considered intersecting after aligning to mirror granularity), it may
happen that request A waits request B in mirror_wait_on_conflicts() and
request B waits for A.

Look at the test for details. Test now dead-locks, that's why it's
disabled. Next commit will fix mirror and enable the test.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210702211636.228981-3-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>