clippy: fix warnings from useless_format lint

Author: cakebaker

src/side_diff.rs

@@ -901,7 +901,7 @@ mod tests {
             let symbol = b'<'; // impossible case, just to use different symbol
             let mut buf = vec![];
             push_output(left_ln, right_ln, symbol, &mut buf, &config).unwrap();
-            assert_eq!(buf, format!("data\t\t\t\t\t\t\t      <\n").as_bytes());
+            assert_eq!(buf, "data\t\t\t\t\t\t\t      <\n".as_bytes());
         }
 
         #[test]
@@ -948,7 +948,7 @@ mod tests {
             let symbol = b' ';
             let mut buf = vec![];
             push_output(left_ln, right_ln, symbol, &mut buf, &config).unwrap();
-            let expected_left = format!("áéíóú\t\t\t\t\t\t\t\t");
+            let expected_left = "áéíóú\t\t\t\t\t\t\t\t";
             let expected_right = "😀😃😄";
             assert_eq!(buf, format!("{expected_left}{expected_right}\n").as_bytes());
         }