Ignore clippy::redundant_clone warning in test

```
warning: redundant clone
   --> tests/waker_ready.rs:192:25
    |
192 |     let w2 = get_waker().clone();
    |                         ^^^^^^^^ help: remove this
    |
    = note: `#[warn(clippy::redundant_clone)]` on by default
note: this value is dropped without further use
   --> tests/waker_ready.rs:192:14
    |
192 |     let w2 = get_waker().clone();
    |              ^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone
```

Author: taiki-e

tests/waker_ready.rs

@@ -175,6 +175,7 @@ fn wake_by_ref() {
     assert_eq!(chan.len(), 0);
 }
 
+#[allow(clippy::redundant_clone)] // This is intentional
 #[test]
 fn clone() {
     future!(f, get_waker, POLL, DROP_F);