Use Order factory to create order in coupon specs

The order is not valid without a store and this way the test
is more reliable, it reflects more closely what happens IRL.

core/spec/models/spree/promotion_handler/coupon_spec.rb

@@ -169,7 +169,7 @@ def expect_adjustment_creation(adjustable:, promotion:, promotion_code: nil)
           end
 
           context "applied alongside another valid promotion " do
-            let!(:order) { Order.create }
+            let!(:order) { create(:order) }
 
             before do
               order.coupon_code = "10off"