fixes bug 902839 - signature summary untested on product_list

@selenamarie r?
cc @brandonsavage

Basically, the tests never tested what would happen if that `product_list` was something. That's because there's nothing in `product_version_builds`.

Perhaps I'm out of my element here and that there's more serious problems to address. 

One bothering thing is if the report_type is `exploitability`, then the `product_list` does not influence the results at all.