Tokio test mock wait

[devensiv]

Clear waiting after completion of the current wait allowing subsequent waits to be registered instead of being discarded.

Motivation

The Mock in tokio-test::io won't work properly with multiple Action::Wait() actions.

Example using Builder::wait()

    let mut mock = Builder::new()
        .wait(Duration::from_secs(10))
        .read(b"hello ")
        .wait(Duration::from_secs(10))
        .read(b"world!")
        .build();

    let mut buf = [0; 256];

    let start = Instant::now();

    let n = mock.read(&mut buf).await.expect("read 1");
    let n = mock.read(&mut buf).await.expect("read 2");

    // `start.elapsed()` will be something around 10s instead of the expected 20s

The first commit in this PR includes two tests regarding the wait issue, one of which multiple_wait is equivalent to the example above and fails without the fix introduced in the second commit of this PR

Solution

The waiting option on Inner was set on the first Action::Wait() but never cleared, causing subsequent Action::Wait() to be discarded because the value of waiting was already in the past.

The fix clears self.waiting when the wait has finished

[Darksonn]

Thank you for your PR. I have an improvement to your test, but otherwise it looks good.

[Darksonn]

LGTM. Thanks!