improves duplicate label test

Author: spencer3035

tests/ui/duplicate-label-E0381-issue-129274.rs renamed to tests/ui/error-codes/E0381-duplicated-label.rs

@@ -1,3 +1,6 @@
+//! Regression test for duplicated label in E0381 error message.
+//!
+//! Issue: <https://github.com/rust-lang/rust/issues/129274>
 fn main() {
  fn test() {
  loop {

tests/ui/duplicate-label-E0381-issue-129274.stderr renamed to tests/ui/error-codes/E0381-duplicated-label.stderr

@@ -1,5 +1,5 @@
 error[E0381]: used binding `blah` is possibly-uninitialized
- --> $DIR/duplicate-label-E0381-issue-129274.rs:8:33
+ --> $DIR/E0381-duplicated-label.rs:11:33
  |
 LL | let blah: Option<String>;
  | ---- binding declared here but left uninitialized