Fix checkstyle test.

The only reason it passed before was due to a bug in the `diff` crate.
The diff was empty even though the contents of the files were different --
namely one string had a trailing newline character while the other didn't.
Now both have a trailing newline character.

src/checkstyle.rs

@@ -33,7 +33,7 @@ where
 {
     if mode == WriteMode::Checkstyle {
         let mut xml_tail = String::new();
-        xml_tail.push_str("</checkstyle>");
+        xml_tail.push_str("</checkstyle>\n");
         write!(out, "{}", xml_tail)?;
     }
     Ok(())