Switch back non_local_definitions lint to allow-by-default

as request T-lang is requesting some major changes in the lint inner
workings in rust-lang#126768#issuecomment-2192634762

(cherry picked from commit 0c0dfb8)

Author: Urgau

tests/rustdoc-ui/doctest/non_local_defs.rs

@@ -6,6 +6,8 @@
 
 #![doc(test(attr(warn(non_local_definitions))))]
 
+#![doc(test(attr(warn(non_local_definitions))))]
+
 //! ```
 //! #[macro_export]
 //! macro_rules! a_macro { () => {} }