refactored testing

src/uebung09/chess/ChessApp.java

@@ -45,6 +45,7 @@ public class ChessApp {
                     case WHITE_QUEEN -> addQueen(Color.white);
                     case BLACK_KNIGHT -> addKnight(Color.black);
                     case WHITE_KNIGHT -> addKnight(Color.white);
+                    case ITERATOR -> board.iterator();
                     default -> System.out.println("Invalid command " + command);
                 }
             }