iotests: Add tests for invalid Quorum @replaces

Add two tests to see that you cannot replace a Quorum child with the
mirror job while the child is in use by a different parent.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200218103454.296704-19-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>