Use #![warn(let_underscore_drop)] in tests.

src/test/ui/lint/let_underscore/let_underscore_drop.rs

@@ -1,5 +1,5 @@
 // check-pass
-// compile-flags: -W let_underscore_drop
+#![warn(let_underscore_drop)]
 
 struct NontrivialDrop;
 

src/test/ui/lint/let_underscore/let_underscore_drop.stderr

@@ -4,7 +4,11 @@ warning: non-binding let on a type that implements `Drop`
 LL |     let _ = NontrivialDrop;
    |     ^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: requested on the command line with `-W let-underscore-drop`
+note: the lint level is defined here
+  --> $DIR/let_underscore_drop.rs:2:9
+   |
+LL | #![warn(let_underscore_drop)]
+   |         ^^^^^^^^^^^^^^^^^^^
 help: consider binding to an unused variable to avoid immediately dropping the value
    |
 LL |     let _unused = NontrivialDrop;