Keep the clippy rules happy in the difficulty test

The level is built with the struct update syntax instead of a field
assignment after Default::default(), which clippy's
field_reassign_with_default rejects.

Co-Authored-By: minimax-m3.1-flash (Claude Code) <[email protected]>

crates/horizon-client/src/save.rs

@@ -5014,8 +5014,10 @@ mod tests {
         assert_eq!(back.difficulty, "hard");
         assert_eq!(back.difficulty_id(), 3);
         // A world whose difficulty is not one of ours is normal.
-        let mut other = Level::default();
-        other.difficulty = "extreme".into();
+        let other = Level {
+            difficulty: "extreme".into(),
+            ..Level::default()
+        };
         assert_eq!(other.difficulty_id(), 2);
         let _ = fs::remove_dir_all(&dir);
     }