Fix sunspot tests run with sql search engine

Commit c14fea9 introduced a regression where `SearchEngine.kind` was always `:sql` in the Sunspot tests. I've reverted that commit and added assertions to prevent the regression in the future.

@bryanstearns Even though I reverted your commit, I'm not in love with the strategy I employed to switch the search engine around in the specs, so please feel free to revisit that area once this PR is merged and the assertions are in place.