Sporadic test failure on `Subscriber`

[tesaguri]

Offending code in the test:

pipitor/src/websub/subscriber.rs

Lines 340 to 350 in a664f9e

	// Subscription renewal.
	// tokio::task::yield_now().await;
	tokio::time::advance(MARGIN).await;
	let (tx, rx) = oneshot::channel();
	let mut tx = Some(tx);
	// FIXME: This line occasionally hangs. If you uncomment `yield_now` above,
	// the hang occurs deterministically.
	let sock = listener.next().timeout().await.unwrap().unwrap();

The tokio::time::advance should trigger a subscription renewal and the hub listener should receive a subscription request, but in fact, the listener occasionally hangs.

If you uncomment the yield_now, the Scheduler's get_next_tick callback seems to fire before calling tokio::time::advance(), which is unexpected since the refresh_time has not passed then.

tokio-rs/tokio#2090 may be relevant, I guess.

[tesaguri]

Workaround for this was made by replacing the usage of pause()/advance() with actual sleep()s (2b2fb9c), but this is definitely not great as it significantly delays the tests.