We should start by checking whether a piece is placed within the boundaries of the 3x3 board:

package com.packtpublishing.tddjava.ch03tictactoe;

import org.junit.Before;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

public class TicTacToeSpec {
  @Rule
  public ExpectedException exception = ExpectedException.none();
  private TicTacToe ticTacToe;

  @Before
  public final void before() {
    ticTacToe = new TicTacToe();
  }
  @Test
  public void whenXOutsideBoardThenRuntimeException() {
    exception.expect(RuntimeException.class);
    ticTacToe.play(5, 2);
  }
} 

In this test, we are defining that RuntimeException is expected when the ticTacToe.play(5, 2) method is invoked. It's a very short and easy test, and making it pass should be easy as well. All we have to do is create the play method and make sure that it throws RuntimeException when the x argument is smaller than 1 or bigger than 3 (the board is 3x3). You should run this test three times. The first time, it should fail because the play method doesn't exist. Once it is added, it should fail because RuntimeException is not thrown. The third time, it should be successful because the code that corresponds with this test is fully implemented.