Fix flaky test (#6131)

* Split test + decrease test timeout

* fmt

* spellcheck

polkadot/node/subsystem-test-helpers/src/lib.rs

@@ -30,6 +30,7 @@ use sp_core::testing::TaskExecutor;
 
 use std::{
 	convert::Infallible,
+	future::Future,
 	pin::Pin,
 	sync::Arc,
 	task::{Context, Poll, Waker},
@@ -391,6 +392,34 @@ macro_rules! arbitrary_order {
 	};
 }
 
+/// Future that yields the execution once and resolves
+/// immediately after.
+///
+/// Useful when one wants to poll the background task to completion
+/// before sending messages to it in order to avoid races.
+pub struct Yield(bool);
+
+impl Yield {
+	/// Returns new `Yield` future.
+	pub fn new() -> Self {
+		Self(false)
+	}
+}
+
+impl Future for Yield {
+	type Output = ();
+
+	fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+		if !self.0 {
+			self.0 = true;
+			cx.waker().wake_by_ref();
+			Poll::Pending
+		} else {
+			Poll::Ready(())
+		}
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;