add_filter("manage_testimonial-edit_columns", "testimonial_edit_columns");
add_action("manage_posts_custom_column",  "testimonial_columns_display", 10, 2);

function testimonial_edit_columns($testimonial_columns){
   $testimonial_columns = array(
      "cb" => "<input type=\"checkbox\" />",
      "title" => "Title",
	  "description" => "Description"
   );
  return $testimonial_columns;
}
 function testimonial_columns_display($testimonial_columns){
 switch ($testimonial_columns)
 {
   case "description":
   the_excerpt();
   break;
 }
}