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);
   |
```

Author: estebank

tests/ui/async-await/pin-needed-to-poll-3.rs

@@ -0,0 +1,25 @@
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
+
+
+struct FutureWrapper<F> {
+    fut: F,
+}
+
+impl<F> Future for FutureWrapper<F>
+where
+    F: Future,
+{
+    type Output = F::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let res = self.fut.poll(cx);
+        //~^ ERROR no method named `poll` found for type parameter `F` in the current scope
+        res
+    }
+}
+
+fn main() {}

tests/ui/async-await/pin-needed-to-poll-3.stderr

@@ -0,0 +1,18 @@
+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);
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.