new_contents: import pytest
  
  from adhocracy4.comments.models import Comment
  from meinberlin.apps.exports import mixins
  
  
  @pytest.mark.django_db
  def test_reply_to_mixin(idea, comment_factory):
      mixin = mixins.ItemExportWithRepliesToMixin()
  
      virtual = mixin.get_virtual_fields({})
      assert 'replies_to' in virtual
  
      comment = comment_factory(content_object=idea)
      reply_comment = comment_factory(content_object=comment)
  
      assert Comment.objects.count() == 2
  
      assert mixin.get_replies_to_data(comment) == ''
      assert mixin.get_replies_to_data(reply_comment) == comment.id
  
subject: add test for new mixin