Avoid Topic.dup to prevent weird test failure

This is an alternative of 65c4b1b50d.

https://buildkite.com/rails/rails/builds/59147#117e9445-23e8-455a-b486-ea0ae9636405/120-129

activerecord/test/cases/serialized_attribute_test.rb

@@ -374,7 +374,7 @@ class SerializedAttributeTest < ActiveRecord::TestCase
   end
 
   def test_serialized_attribute_works_under_concurrent_initial_access
-    model = Topic.dup
+    model = Class.new(Topic)
 
     topic = model.create!
     topic.update group: "1"