Add reusable action adjustment trait on Promotion factory

Currently, we do not have a factory for Promotion::Actions and the
existing adjustment traits are not flexible. This trait will be useful,
for more flexiblity when testing non out-of-the-box Promotion Actions
and different calculators.

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

@@ -27,6 +27,22 @@
       end
     end
 
+    trait :with_adjustable_action do
+      transient do
+        preferred_amount { 10 }
+        calculator_class { Spree::Calculator::FlatRate }
+        promotion_action_class { Spree::Promotion::Actions::CreateItemAdjustments }
+      end
+
+      after(:create) do |promotion, evaluator|
+        calculator = evaluator.calculator_class.new
+        calculator.preferred_amount = evaluator.preferred_amount
+        evaluator.promotion_action_class.create!(calculator: calculator, promotion: promotion)
+      end
+    end
+
+    factory :promotion_with_action_adjustment, traits: [:with_adjustable_action]
+
     trait :with_line_item_adjustment do
       transient do
         adjustment_rate { 10 }

core/spec/lib/spree/core/testing_support/factories/promotion_factory_spec.rb

@@ -11,6 +11,12 @@
     it_behaves_like 'a working factory'
   end
 
+  describe 'promotion with action adjustment' do
+    let(:factory) { :promotion_with_action_adjustment }
+
+    it_behaves_like 'a working factory'
+  end
+
   describe 'promotion with item adjustment' do
     let(:factory) { :promotion_with_item_adjustment }