Add ability to specify variant for order

I will need this in the following commit. For existing specs where no variant is specified, a product with variant will be created (I think this is what the line item factory would have done before).

spec/factories/order_factory.rb

@@ -141,15 +141,16 @@ FactoryBot.define do
   factory :order_with_totals_and_distribution, parent: :order_with_distributor do
     transient do
       shipping_fee { 3 }
+      product { variant&.product || create(:simple_product) }
+      variant { product.variants.first }
     end
 
     order_cycle { create(:simple_order_cycle) }
 
     after(:create) do |order, proxy|
-      product = create(:simple_product)
       create(:line_item_with_shipment, shipping_fee: proxy.shipping_fee,
                                        order:,
-                                       product:)
+                                       variant: proxy.variant)
       order.reload
     end