Fix specs to reflect changes in core

As done in the main repository, there are some changes
needed in the specs after some changes happened in the
StockQuantifier.

See solidusio/solidus#4291 for more information.

templates/spec/system/checkout_confirm_insufficient_stock_spec.rb

@@ -10,7 +10,7 @@
 
     let(:order) { Spree::TestingSupport::OrderWalkthrough.up_to(:payment) }
     let(:order_product) { order.products.first }
-    let(:order_stock_item) { order_product.stock_items.first }
+    let(:order_stock_item) { order.line_items.first.variant.stock_items.first }
 
     before do
       order_stock_item.update! backorderable: false
@@ -25,6 +25,7 @@
         before do
           visit spree.checkout_state_path(:confirm)
           order_stock_item.set_count_on_hand(0)
+          order.line_items.first.variant.stock_items.reload
         end
 
         it 'redirects to cart page and shows an unavailable product message' do
@@ -44,6 +45,7 @@
         before do
           visit spree.checkout_state_path(:confirm)
           order_stock_item.set_count_on_hand(0)
+          order.line_items.first.variant.stock_items.reload
         end
 
         it "redirects to the address checkout page and shows an availability changed message" do