Fix serialization of expected outcomes in test driver.

The outcomes are saved as a set and need to be converted into
a list to be json-serializable.

TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/710073002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25246 ce2b1a6d-e550-0410-aec6-3dcde31c8c00