Improve display of assert_public_ty assertion failures (#13577)

While working on #13576 I noticed
that it was really hard to tell which assertion failed in some of these
test cases. This could be expanded to elsewhere, but I've heard this
test suite format won't be around for long?

crates/red_knot_python_semantic/src/types/infer.rs

@@ -2948,7 +2948,11 @@ mod tests {
         let file = system_path_to_file(db, file_name).expect("Expected file to exist.");
 
         let ty = global_symbol_ty(db, file, symbol_name);
-        assert_eq!(ty.display(db).to_string(), expected);
+        assert_eq!(
+            ty.display(db).to_string(),
+            expected,
+            "Mismatch for symbol '{symbol_name}' in '{file_name}'"
+        );
     }
 
     fn assert_scope_ty(