iotests: Test two stream jobs in a single iothread

iotests: Test two stream jobs in a single iothread

This tests two parallel stream jobs that will complete around the same
time and run on two different disks in the same iothreads. It is loosely
based on the bug report at https://issues.redhat.com/browse/RHEL-1761.

For me, this test hangs reliably with the originally reported bug in
blk_remove_bs(). After fixing it, it intermittently hangs for the bugs
fixed after it, missing AioContext unlocking in bdrv_graph_wrunlock()
and in stream_prepare(). The deadlocks seem to happen more frequently
when the test directory is on tmpfs.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20231115172012.112727-5-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>