Fix CI failure due to remaining tagging records

`TRUNCATE TABLE posts` also resets `AUTO_INCREMENT`. If newly created a
post, it is wrongly associated with remaining tagging records.
To un-associate remaining tagging record, use `post.create_tagging!`
instead.

Fixes #35751.

activerecord/test/cases/associations/eager_load_includes_full_sti_class_test.rb

@@ -21,7 +21,7 @@ module PolymorphicFullStiClassNamesSharedTest
     ActiveRecord::Base.store_full_sti_class = store_full_sti_class
 
     post = Namespaced::Post.create(title: "Great stuff", body: "This is not", author_id: 1)
-    @tagging = Tagging.create(taggable: post)
+    @tagging = post.create_tagging!
   end
 
   def teardown