Check polymorphic type in validations

* Previously, some models had "duplicated" `belongs_to` associations, which were now removed (and replaced by the `include Creation`).

app/models/testrun.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 class Testrun < ApplicationRecord
+  include Creation
   belongs_to :file, class_name: 'CodeOcean::File', optional: true
   belongs_to :submission
   belongs_to :testrun_execution_environment, optional: true, dependent: :destroy