[board] Implement a bunch of sight tests

board/src/position/position.rs

@@ -236,6 +236,12 @@ impl Position {
             .next()
             .unwrap();
         sight_of_opposing_player.is_set(king_square)
+}
+
+#[cfg(test)]
+impl Position {
+    pub(crate) fn test_set_en_passant_square(&mut self, square: Square) {
+        self.en_passant_square = Some(square);
     }
 }