Whole subsystem test for new availability-distribution (#2552)

* WIP: Whole subsystem test.

* New tests compile.

* Avoid needless runtime queries for no validator nodes.

* Make tx and rx publicly accessible in virtual overseer.

This simplifies mocking in some cases, as tx can be cloned, but rx can
not.

* Whole subsystem test working.

* Update node/network/availability-distribution/src/session_cache.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Update node/network/availability-distribution/src/session_cache.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Document better what `None` return value means.

* Get rid of BitVec dependency.

* Update Cargo.lock

* Hopefully fixed implementers guide build.

Co-authored-by: Andronik Ordian <write@reusable.software>

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

@@ -51,6 +51,13 @@ enum SinkState<T> {
 /// The sink half of a single-item sink that does not resolve until the item has been read.
 pub struct SingleItemSink<T>(Arc<Mutex<SinkState<T>>>);
 
+// Derive clone not possible, as it puts `Clone` constraint on `T` which is not sensible here.
+impl<T> Clone for SingleItemSink<T> {
+	fn clone(&self) -> Self {
+		Self(self.0.clone())
+	}
+}
+
 /// The stream half of a single-item sink.
 pub struct SingleItemStream<T>(Arc<Mutex<SinkState<T>>>);
 
@@ -213,8 +220,14 @@ impl<M: Send + 'static, S: SpawnNamed + Send + 'static> SubsystemContext
 
 /// A handle for interacting with the subsystem context.
 pub struct TestSubsystemContextHandle<M> {
-	tx: SingleItemSink<FromOverseer<M>>,
-	rx: mpsc::UnboundedReceiver<AllMessages>,
+	/// Direct access to sender of messages.
+	///
+	/// Useful for shared ownership situations (one can have multiple senders, but only one
+	/// receiver.
+	pub tx: SingleItemSink<FromOverseer<M>>,
+
+	/// Direct access to the receiver.
+	pub rx: mpsc::UnboundedReceiver<AllMessages>,
 }
 
 impl<M> TestSubsystemContextHandle<M> {