[position] Fix broken tests build

The make_move tests were trying to access the last capture piece directly from a
slice of captures pieces (implementation prior to CapturesList). Implement
CapturesList::last() to return an Option<&Piece> and use it in the tests.

position/src/position/captures.rs

@@ -8,6 +8,11 @@ use chessfriend_core::{Color, Piece};
 pub(crate) struct CapturesList([Vec<Piece>; Color::NUM]);
 
 impl CapturesList {
+    #[cfg(test)]
+    pub fn last(&self, color: Color) -> Option<&Piece> {
+        self.0[color as usize].last()
+    }
+
     pub fn push(&mut self, color: Color, piece: Piece) {
         self.0[color as usize].push(piece);
     }

position/src/position/make_move.rs

@@ -407,7 +407,7 @@ mod tests {
 
         assert_eq!(pos.get_piece(Square::C2), None);
         assert_eq!(pos.get_piece(Square::F5), Some(piece!(White Bishop)));
-        assert_eq!(pos.captures[Color::White as usize][0], piece!(Black Rook));
+        assert_eq!(pos.captures.last(Color::White), Some(&piece!(Black Rook)));
         assert_eq!(pos.board.active_color, Color::Black);
         assert_eq!(pos.board.half_move_clock, 0);
 
@@ -444,7 +444,7 @@ mod tests {
             None,
             "capture target pawn not removed"
         );
-        assert_eq!(pos.captures[Color::Black as usize][0], piece!(White Pawn));
+        assert_eq!(pos.captures.last(Color::Black), Some(&piece!(White Pawn)));
 
         Ok(())
     }