Now that we have declared what the default response is (No winner), it's time to start working on different winning conditions:

@Test
public void whenPlayAndWholeHorizontalLineThenWinner() {
  ticTacToe.play(1, 1); // X
  ticTacToe.play(1, 2); // O
  ticTacToe.play(2, 1); // X
  ticTacToe.play(2, 2); // O
  String actual = ticTacToe.play(3, 1); // X
  assertEquals("X is the winner", actual);
}