[Partial fix] Graph cluster option acceptance test

The current version of Graph::Writer::Dot doesn't
generate a list of subgraphs
(sended pull request to Graph-ReadWrite:
https://github.com/neilbowers/Graph-ReadWrite/pull/1)

Signed-off-by: Lucas Kanashiro <lucas-kanashiro@hotmail.com>
Signed-off-by: Alexandre Barbosa <alexandreab@live.com>