Add test for incorrect pinning suggestion

The following suggestion is incorrect, as it doesn't account for the binding:

```
error[E0599]: no method named `poll` found for type parameter `F` in the current scope
  --> $DIR/pin-needed-to-poll-3.rs:19:28
   |
LL | impl<F> Future for FutureWrapper<F>
   |      - method `poll` not found for this type parameter
...
LL |         let res = self.fut.poll(cx);
   |                            ^^^^ method not found in `F`
   |
help: consider pinning the expression
   |
LL ~         let res = let mut pinned = std::pin::pin!(self.fut);
LL ~         pinned.as_mut().poll(cx);
   |
```

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