[board] Remove the intermediate String when building a diagram representation of a Position

Also ignore the tests in position::diagram_formatter::tests. These don't assert
anything; they're just there for visual spot check.

board/src/position/diagram_formatter.rs

@@ -1,7 +1,7 @@
 // Eryn Wells <eryn@erynwells.me>
 
 use crate::{File, Position, Rank, Square};
-use std::{fmt, fmt::Write};
+use std::fmt;
 
 pub struct DiagramFormatter<'a>(&'a Position);
 
@@ -13,38 +13,36 @@ impl<'a> DiagramFormatter<'a> {
 
 impl<'a> fmt::Display for DiagramFormatter<'a> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let mut output = String::new();
-
-        output.push_str("  +-----------------+\n");
+        write!(f, "  +-----------------+\n")?;
 
         for rank in Rank::ALL.iter().rev() {
-            write!(output, "{} | ", rank)?;
+            write!(f, "{rank} | ")?;
 
             for file in File::ALL.iter() {
                 let square = Square::from_file_rank(*file, *rank);
                 match self.0.piece_on_square(square) {
-                    Some(placed_piece) => write!(output, "{} ", placed_piece.piece())?,
-                    None => output.push_str(". "),
+                    Some(placed_piece) => write!(f, "{} ", placed_piece.piece())?,
+                    None => write!(f, ". ")?,
                 }
             }
 
-            output.push_str("|\n");
+            write!(f, "|\n")?;
         }
 
-        output.push_str("  +-----------------+\n");
-        output.push_str("    a b c d e f g h\n");
+        write!(f, "  +-----------------+\n")?;
+        write!(f, "    a b c d e f g h\n")?;
 
-        write!(f, "{}", output)
+        Ok(())
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::piece::{Color, Piece};
-    use crate::{Position, PositionBuilder};
+    use crate::Position;
 
     #[test]
+    #[ignore]
     fn empty() {
         let pos = Position::empty();
         let diagram = DiagramFormatter(&pos);
@@ -52,15 +50,15 @@ mod tests {
     }
 
     #[test]
+    #[ignore]
     fn one_king() {
-        let pos = PositionBuilder::new()
-            .place_piece(piece!(Black King on H3))
-            .build();
+        let pos = position![Black King on H3];
         let diagram = DiagramFormatter(&pos);
         println!("{}", diagram);
     }
 
     #[test]
+    #[ignore]
     fn starting() {
         let pos = Position::starting();
         let diagram = DiagramFormatter(&pos);