Ignore clippy::let_underscore_future lint

```
warning: non-binding `let` on a future
  --> futures-util/src/future/select_all.rs:61:17
   |
61 |                 let _ = self.inner.swap_remove(idx);
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider awaiting the future or dropping explicitly with `std::mem::drop`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_future
   = note: `#[warn(clippy::let_underscore_future)]` on by default

warning: non-binding `let` on a future
   --> futures/tests/async_await_macros.rs:385:5
    |
385 |     let _ = async { join!(async {}, async {}) };
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider awaiting the future or dropping explicitly with `std::mem::drop`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_future
    = note: `#[warn(clippy::let_underscore_future)]` on by default

warning: non-binding `let` on a future
   --> futures/tests/async_await_macros.rs:390:5
    |
390 |     let _ = async { try_join!(async { Ok::<(), ()>(()) }, async { Ok::<(), ()>(()) },) };
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider awaiting the future or dropping explicitly with `std::mem::drop`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_future
```

futures-util/src/future/select_all.rs

@@ -58,6 +58,7 @@ impl<Fut: Future + Unpin> Future for SelectAll<Fut> {
         });
         match item {
             Some((idx, res)) => {
+                #[allow(clippy::let_underscore_future)]
                 let _ = self.inner.swap_remove(idx);
                 let rest = mem::take(&mut self.inner);
                 Poll::Ready((res, idx, rest))

futures/tests/async_await_macros.rs

@@ -380,11 +380,13 @@ fn try_join_size() {
     assert_eq!(mem::size_of_val(&fut), 48);
 }
 
+#[allow(clippy::let_underscore_future)]
 #[test]
 fn join_doesnt_require_unpin() {
     let _ = async { join!(async {}, async {}) };
 }
 
+#[allow(clippy::let_underscore_future)]
 #[test]
 fn try_join_doesnt_require_unpin() {
     let _ = async { try_join!(async { Ok::<(), ()>(()) }, async { Ok::<(), ()>(()) },) };