Complete Castling Tests Comprehensiveness

[frozenfrank]

Overview

Naive implements of Castling could get away with it because of spotty test coverage.

Discussion

The Castling tests, along with other tests, were designed to give us a "good enough" assurance that the code was implemented properly. Each of the conditions are tested, but most are only tested from a single perspective.

Reasonable implementations of chess would work if all of these tests work, but students do all sorts of ugly, horrific, and unreasonable things to their code. We should make sure we test it properly.

This visualization was first introduced when the castling tests were most recently factored in #13.

Visualization

The current state of the tested conditions is as follows. Notice the significant holes.

Stated Condition	Tested for "White"	Tested for "Black"
King can castle correctly	✅ castleWhite	✅ castleBlack
Neither the King nor Rook have moved since the game started	✅ noCastleAfterMove	❌
There are no pieces between the King and the Rook	✅ noCastleThroughPieces	❌
The King does not start in Check	❌	✅ noCastleFromCheck
[The King] does not cross a square on which it would be in Check	❌	✅ noCastleThroughCheck
[The King] is not in Check after castling	✅ noCastleIntoCheck	❌