Add test current_player returns B on black s turn
Add test current_player returns W on white s turn
Add test current_opponent returns W on black's turn
Add test current_opponent returns B on white's turn
Extract constants: BLACK and WHITE
Add test player_at returns the player at the given position
Add test player_at returns None when no player at the given position
Extract method Game._get_pieces
Add test left_of D4 is C4
Add test left_of A0 is None
Extract class constant Game._COLUMNS
Add test right_of C4 is D4
Add test right_of H8 is None
Extract method Game._column
Extract method Game._column_add
Extract class Column
Add test top_of D4 is D3
Add test top_of A0 is None
Add test bottom_of D4 is D5
Add test bottom of_H8 is None