Refactor ChessBoard tests.

I shuffled the position of the tests around and renamed some of them for
readability.
Rename ChessBoard tests.
Rename makeMove() to forceMove().

The name forceMove() makes it clear that it will work regardless of a
ChessPiece's movement pattern.
Rename checkValidMove() to isValidMove().

isValidMove() is more consistant, it matches the similar functions in
the various ChessPiece classes.