iotests/283: Check that finalize drops backup-top

Without any of HEAD^ or HEAD^^ applied, qemu will most likely crash on
the qemu-io invocation, for a variety of immediate reasons.  The
underlying problem is generally a use-after-free access into
backup-top's BlockCopyState.

With only HEAD^ applied, qemu-io will run into an EIO (which is not
capture by the output, but you can see that the qemu-io invocation will
be accepted (i.e., qemu-io will run) in contrast to the reference
output, where the node name cannot be found), and qemu will then crash
in query-named-block-nodes: bdrv_get_allocated_file_size() detects
backup-top to be a filter and passes the request through to its child.
However, after bdrv_backup_top_drop(), that child is NULL, so the
recursive call crashes.

With HEAD^^ applied, this test should pass.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210219153348.41861-4-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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