Finally, there is the last possible winning condition to tackle:
@Test
public void whenPlayAndBottomTopDiagonalLineThenWinner() {
ticTacToe.play(1, 3); // X
ticTacToe.play(1, 1); // O
ticTacToe.play(2, 2); // X
ticTacToe.play(1, 2); // O
String actual = ticTacToe.play(3, 1); // X
assertEquals("X is the winner", actual);
}
The player wins when the whole diagonal line from the bottom-left to top-right is occupied by his pieces.