Fix order walkthrough for iso uniqueness

ISO is now a unique field on countries which means that creating two
countries errors. Creating the country here broke tests in some
extensions including solidus_starter_frontend.

Author: AlistairNorman

core/lib/spree/testing_support/order_walkthrough.rb

@@ -10,7 +10,7 @@ def self.up_to(state, user: nil)
       def up_to(state, user: nil)
         # Need to create a valid zone too...
         @zone = ::FactoryBot.create(:zone)
-        @country = ::FactoryBot.create(:country)
+        @country = Spree::Country.find_by(iso: "US") || ::FactoryBot.create(:country)
         @state = ::FactoryBot.create(:state, country: @country)
 
         @zone.members << Spree::ZoneMember.create(zoneable: @country)