iotests: Test active commit with iothread and background I/O

iotests: Test active commit with iothread and background I/O

This is a better regression test for the bugs hidden by commit 80fc5d26
('graph-lock: Disable locking for now'). With that commit reverted, it
hangs instantaneously and reliably for me.

It is important to have a reliable test like this, because the following
commits will set out to fix the actual root cause of the deadlocks and
then finally revert commit 80fc5d26, which was only a stopgap solution.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230605085711.21261-2-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>