qemu-iotests: New test case in 061

Add one test case for zero cluster expansion on qcow2 version downgrade
in shared L2 tables (i.e., L2 tables with a refcount > 1) and one for
zero expansion on backed clusters in shared L2 tables.

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