Refactor Promotion factory

Noteably, keep existing transient arguments to not cause breaking specs
for stores. Also, by using the new  with_adjustable_action trait, they
automatically become more flexible due to its transient arguments.

core/lib/spree/testing_support/factories/promotion_factory.rb

@@ -22,8 +22,12 @@
     end
 
     trait :with_action do
-      after(:create) do |promotion, _evaluator|
-        promotion.actions << Spree::Promotion::Actions::CreateAdjustment.new
+      transient do
+        promotion_action_class { Spree::Promotion::Actions::CreateAdjustment }
+      end
+
+      after(:create) do |promotion, evaluator|
+        promotion.actions << evaluator.promotion_action_class.new
       end
     end
 
@@ -48,11 +52,8 @@
         adjustment_rate { 10 }
       end
 
-      after(:create) do |promotion, evaluator|
-        calculator = Spree::Calculator::FlatRate.new
-        calculator.preferred_amount = evaluator.adjustment_rate
-        Spree::Promotion::Actions::CreateItemAdjustments.create!(calculator: calculator, promotion: promotion)
-      end
+      with_adjustable_action
+      preferred_amount { adjustment_rate }
     end
 
     factory :promotion_with_item_adjustment, traits: [:with_line_item_adjustment]
@@ -68,14 +69,11 @@
         weighted_order_adjustment_amount { 10 }
       end
 
-      after(:create) do |promotion, evaluator|
-        calculator = Spree::Calculator::FlatRate.new
-        calculator.preferred_amount = evaluator.weighted_order_adjustment_amount
-        action = Spree::Promotion::Actions::CreateAdjustment.create!(calculator: calculator)
-        promotion.actions << action
-        promotion.save!
-      end
+      with_adjustable_action
+      preferred_amount { weighted_order_adjustment_amount }
+      promotion_action_class { Spree::Promotion::Actions::CreateAdjustment }
     end
+
     factory :promotion_with_order_adjustment, traits: [:with_order_adjustment]
 
     trait :with_item_total_rule do