Fix insufficient stock spec

This has to make use of a helper method instead of a memoized
`let` now, because the object graph in the spec is otherwise different
than the one in the tested code.

frontend/spec/features/checkout_confirm_insufficient_stock_spec.rb

@@ -10,11 +10,16 @@
 
     let(:order) { Spree::TestingSupport::OrderWalkthrough.up_to(:payment, user: user) }
     let(:order_product) { order.products.first }
-    let(:order_stock_item) { order_product.stock_items.first }
+
+    # This can't be memoized now in a `let` block, otherwise we
+    # get a different stock item in the object graph than in the spec setup
+    def stock_item
+      order.line_items.first.variant.stock_items.first
+    end
 
     before do
-      order_stock_item.update! backorderable: false
-      order_stock_item.set_count_on_hand(1)
+      stock_item.update! backorderable: false
+      stock_item.set_count_on_hand(1)
       allow_any_instance_of(Spree::CheckoutController).to receive_messages(current_order: order)
       allow_any_instance_of(Spree::CheckoutController).to receive_messages(try_spree_current_user: user)
       allow_any_instance_of(Spree::OrdersController).to receive_messages(try_spree_current_user: user)
@@ -24,7 +29,7 @@
       context 'when the customer is on the confirm page and the availabilty drops to zero' do
         before do
           visit spree.checkout_state_path(:confirm)
-          order_stock_item.set_count_on_hand(0)
+          stock_item.set_count_on_hand(0)
         end
 
         it 'redirects to cart page and shows an unavailable product message' do
@@ -43,7 +48,7 @@
       context 'when the customer is on the confirm page and the availabilty drops to zero' do
         before do
           visit spree.checkout_state_path(:confirm)
-          order_stock_item.set_count_on_hand(0)
+          stock_item.set_count_on_hand(0)
         end
 
         it "redirects to the address checkout page and shows an availability changed message" do