Write more tests

[jamesqo]

• [x] Pawn on 7th rank should not cause bounds error (regression)
• [ ] Castling scenarios
  • [ ] Castling should not be possible if there are pieces between the king and the rook (regression)
• [x] Promotion should happen iff pawn moves to back rank (regression)
• [x] Assert.Throws<AnParseException> tests should be moved from StateTests to MoveTests, or these exceptions should be wrapped
• [ ] En passant tests for ApplyMove, IsMovePossible / GetPossibleDestinations, ParseFen, etc. once the functionality is implemented