fn pending_once(self) -> PendingOnce<Self>
    where
        Self: Sized,
    {
        PendingOnce::new(self)
    }
Introduces one [`Poll::Pending`](futures_core::task::Poll::Pending)
before polling the given future.

# Examples

```
use core::pin::pin;

use futures::task::Poll;
use futures::future::FutureExt;
use futures_test::task::noop_context;
use futures_test::future::FutureTestExt;

let future = (async { 5 }).pending_once();
let mut future = pin!(future);

let mut cx = noop_context();

assert_eq!(future.poll_unpin(&mut cx), Poll::Pending);
assert_eq!(future.poll_unpin(&mut cx), Poll::Ready(5));
```
futures-test/src/future/mod.rs