test that running nufmt twice produces the same output (#40)

original discussion on Discord:
https://discord.com/channels/601130461678272522/1117921521520873623/1123167419951763466

cc/ @AucaCoyan

src/lib.rs

@@ -41,11 +41,14 @@ pub fn format_string(input_string: &String, config: &Config) -> String {
 mod test {
  use super::*;
 
+ /// test that
+ /// 1. formatting the input gives the expected result
+ /// 2. formatting the output of `nufmt` a second time does not change the content
  fn run_test(input: &str, expected: &str) {
- assert_eq!(
- expected.to_string(),
-  format_string(&input.to_string(), &Config::default())
- );
+ let formatted = format_string(&input.to_string(), &Config::default());
+
+ assert_eq!(expected.to_string(), formatted);
+ assert_eq!(formatted, format_string(&formatted, &Config::default()));
  }
 
  #[test]