check content of the error message

in suffix_items_cannot_be_followed_by_a_list

src/book/summary.rs

@@ -747,6 +747,8 @@ mod tests {
         let got = parser.parse_affix(false);
 
         assert!(got.is_err());
+        let error_message = got.err().unwrap().to_string();
+        assert_eq!(error_message, "failed to parse SUMMARY.md line 2, column 1: Suffix chapters cannot be followed by a list");
     }
 
     #[test]