Adds assertion to PieceTest::testCheckmate

+ Stalemate is not the same as checkmate
Satisfies PieceTest::testCheckmate

+ Stalemate is not the same as checkmate
Adds PieceTest::testStalemate
Satisfies PieceTest::testStalemate
Adds assertion to PieceTest::testStalemate

+ King should not be in stalemate
Satisfies PieceTest::testStalemate

+ King should not be in stalemate