The `Spree::Admin::OrdersController`'s test file quite long (as well as
the controller itself) but it'll grow more in the upcoming commits.
That's just a symptom of that controller having too many
responsibilities. It does much more than CRUD on `Spree::Order` (the
Rails convention).
Things like invoices are an entity on their own and would better fit
into a `InvoicesController`. Hopefully, splitting up the tests is hint
for the next dev to do that.
