iotests/{024, 271}: add testcases for qemu-img rebase

iotests/{024, 271}: add testcases for qemu-img rebase

As the previous commit changes the logic of "qemu-img rebase" (it's using
write alignment now), let's add a couple more test cases which would
ensure it works correctly.  In particular, the following scenarios:

024: add test case for rebase within one backing chain when the overlay
     cluster size > backings cluster size;
271: add test case for rebase images that contain subclusters.  Check
     that no extra allocations are being made.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20230919165804.439110-7-andrey.drobyshev@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>