iotests/graph-changes-while-io: add test case with removal of lower snapshot

iotests/graph-changes-while-io: add test case with removal of lower snapshot

This case is catching potential deadlock which takes place when job-dismiss
is issued when I/O requests are processed in a separate iothread.

See https://mail.gnu.org/archive/html/qemu-devel/2025-04/msg04421.html

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
[FE: re-use top image and rename snap1->mid as suggested by Kevin Wolf
     remove image file after test as suggested by Kevin Wolf
     add type annotation for function argument to make mypy happy]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-22-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>