We should also check whether there is a win by filling the vertical line:
@Test
public void whenPlayAndWholeVerticalLineThenWinner() {
ticTacToe.play(2, 1); // X
ticTacToe.play(1, 1); // O
ticTacToe.play(3, 1); // X
ticTacToe.play(1, 2); // O
ticTacToe.play(2, 2); // X
String actual = ticTacToe.play(1, 3); // O
assertEquals("O is the winner", actual);
}
The player wins when the whole vertical line is occupied by his pieces.