Increase test coverage of `build_lots_and_categories_link_tree`
Increase test coverage of `build_lots_and_categories_link_tree`

We have no direct tests which show what it should be returning.

alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend