Bring adjustment factory from spree_core

spec/factories/adjustment_factory.rb

@@ -0,0 +1,9 @@
+FactoryBot.define do
+  factory :adjustment, class: Spree::Adjustment do
+    association(:adjustable, factory: :order)
+    amount 100.0
+    label 'Shipping'
+    association(:source, factory: :shipment)
+    eligible true
+  end
+end