Sort OC report orders by completed_at

This is then consistent with the ordering we use to list orders in
/admin, which is more useful. As a result, the test is also more robust.

lib/open_food_network/order_cycle_management_report.rb

@@ -65,7 +65,7 @@ module OpenFoodNetwork
 
     def orders
       if FeatureToggle.enabled?(:customer_balance, @user)
-        search_result = search.result.order(:id)
+        search_result = search.result.order(:completed_at)
         orders_with_balance = OutstandingBalance.new(search_result).
           query.
           select('spree_orders.*')

spec/lib/open_food_network/order_cycle_management_report_spec.rb

@@ -43,11 +43,10 @@ module OpenFoodNetwork
           end
 
           it 'orders them by id' do
-            result = instance_double(ActiveRecord::Relation)
-            allow_any_instance_of(Ransack::Search).to receive(:result).and_return(result)
-            expect(result).to receive(:order).with(:id) { Spree::Order.none }
+            order1 = create(:order, completed_at: 1.day.ago, state: 'complete')
+            order2 = create(:order, completed_at: 2.days.ago, state: 'complete')
 
-            subject.orders
+            expect(subject.orders.pluck(:id)).to eq([order2.id, order1.id])
           end
         end