[board] A small set of unit tests for Zobrist hashes on Board

A few tests to ensure the zobrist hash changes when changes are made to the Board.
This set isn't exhaustive.

board/src/board.rs

@@ -325,7 +325,7 @@ impl Board {
 mod tests {
     use super::*;
     use crate::test_board;
-    use chessfriend_core::piece;
+    use chessfriend_core::{piece, random::RandomNumberGenerator};
 
     #[test]
     fn get_piece_on_square() {
@@ -335,4 +335,55 @@ mod tests {
 
         assert_eq!(board.get_piece(Square::F7), Some(piece!(Black Bishop)));
     }
+
+    // MARK: - Zobrist Hashing
+
+    fn test_state() -> ZobristState {
+        let mut rng = RandomNumberGenerator::default();
+        ZobristState::new(&mut rng)
+    }
+
+    #[test]
+    fn zobrist_hash_set_for_empty_board() {
+        let state = Arc::new(test_state());
+        let board = Board::empty(Some(state.clone()));
+        let hash = board.zobrist_hash();
+        assert_eq!(hash, Some(0));
+    }
+
+    #[test]
+    fn zobrist_hash_set_for_starting_position_board() {
+        let state = Arc::new(test_state());
+        let board = Board::starting(Some(state.clone()));
+        let hash = board.zobrist_hash();
+        assert!(hash.is_some());
+    }
+
+    #[test]
+    fn zobrist_hash_updated_when_changing_active_color() {
+        let state = Arc::new(test_state());
+
+        let mut board = Board::empty(Some(state.clone()));
+        board.set_active_color(Color::Black);
+
+        // Just verify that the value is real and has changed. The actual value
+        // computation is covered by the tests in zobrist.rs.
+        let hash = board.zobrist_hash();
+        assert!(hash.is_some());
+        assert_ne!(hash, Some(0));
+    }
+
+    #[test]
+    fn zobrist_hash_updated_when_changing_en_passant_target() {
+        let state = Arc::new(test_state());
+
+        let mut board = Board::empty(Some(state.clone()));
+        board.set_en_passant_target(Square::C3);
+
+        // Just verify that the value is real and has changed. The actual value
+        // computation is covered by the tests in zobrist.rs.
+        let hash = board.zobrist_hash();
+        assert!(hash.is_some());
+        assert_ne!(hash, Some(0));
+    }
 }