Include fixtures to prevent foreign key violation.

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -937,6 +937,8 @@ def test_reflect_the_most_recent_change
 end
 
 class BelongsToWithForeignKeyTest < ActiveRecord::TestCase
+  fixtures :authors, :author_addresses
+
   def test_destroy_linked_models
     address = AuthorAddress.create!
     author = Author.create! name: "Author", author_address_id: address.id