tests: Fix non_local_definitions warning

```
error: non-local `impl` definition, they should be avoided as they go against expectation
   --> tests/pinned_drop.rs:140:17
    |
140 | /                 impl S {
141 | |                     fn _f(self) -> Self {
142 | |                         self
143 | |                     }
144 | |                 }
    | |_________________^
    |
    = help: move this `impl` block outside the of the current function `__drop_inner` and up 3 bodies
    = note: an `impl` definition is non-local if it is nested inside an item and neither the type nor the trait are at the same nesting level as the `impl` block
    = note: one exception to the rule are anon-const (`const _: () = { ... }`) at top-level module and anon-const at the same nesting as the trait or type
    = note: this lint may become deny-by-default in the edition 2024 and higher, see the tracking issue <rust-lang/rust#120363>
    = note: `-D non-local-definitions` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(non_local_definitions)]`
```

Author: taiki-e

tests/pinned_drop.rs

@@ -137,6 +137,8 @@ fn self_inside_macro_containing_fn() {
     impl PinnedDrop for S {
         fn drop(self: Pin<&mut Self>) {
             mac!({
+                #[allow(dead_code)]
+                struct S(());
                 impl S {
                     fn _f(self) -> Self {
                         self