Add repos from user_public_repos to the user's search context.

Created by: arussellsaw

This PR hooks up the user_public_repos table to the user's search context, doing a concurrent ListByUser when fetching repos to resolve for the search context. I also slightly expanded the testing on user search contexts to assert that we're getting back the repos we expect from the resolver, and i also replaced a few usages of database.GlobalSomething with an injected dbutil.DB