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/network/availability-distribution/src/error.rs

@@ -57,6 +57,10 @@ pub enum Error {
 	/// We tried accessing a session that was not cached.
 	#[error("Session is not cached.")]
 	NoSuchCachedSession,
+	
+	/// We tried reporting bad validators, although we are not a validator ourselves.
+	#[error("Not a validator.")]
+	NotAValidator,
 
 	/// Requester stream exhausted.
 	#[error("Erasure chunk requester stream exhausted")]