Allow unused fields in some tests

The dead_code lint was previously eroneously missing those.
Since this lint bug has been fixed, the unused fields need
to be marked #[allow(dead_code)].

Author: krtab

tests/ui/impl-not-adjacent-to-type.rs

@@ -3,6 +3,7 @@
 mod foo {
     pub struct Point {
         pub x: i32,
+        #[allow(dead_code)]
         pub y: i32,
     }
 }

tests/ui/union/union-macro.rs

@@ -17,6 +17,7 @@ macro_rules! duplicate {
 
 duplicate! {
     pub union U {
+        #[allow(dead_code)]
         pub a: u8
     }
 }