Fix some bugs in ChessBoard#print
Test if the board is printed correctly
Check if board is properly initialized
Fix ChessBoard#move
Test ChessBoard#move
Fix bugs in ChessBoard#king_remains_safe_after_move? and King#attacked_by_other?
Test ChessBoard#king_remains_safe_after_move?
Test ChessBoard#out_of_the_board? and ChessBoard#color_of_piece_on
Modify ChessBoard#king_of
Test ChessBoard#king_of
Test ChessBoard#empty?
Test ChessBoard#piece_on
Test ChessBoard#pieces_of_the_same_color?
Fix some bugs in Pawn#any_moves?, Knight#any_moves? and ChessBoard#king_remains_safe_after_move?
Test ChessBoard#any_valid_moves_for_player_on_turn?