Rather than modifying the fixture, perhaps create a textarea and append; something like:

    test('Height does not increase once textarea max-height is reached', function() {
      var maxHeight = 200,
          text = 'Hello world!\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n',
          $textarea = $('<textarea />').appendTo('#qunit-fixture');

      $textarea.expanding();

      $textarea.val(text).trigger('input');
      
      equal($textarea.outerHeight(), maxHeight);
      equal($textarea.height(), $textarea.siblings('pre').outerHeight());
    });