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/Cargo.lock

@@ -5153,6 +5153,7 @@ version = "0.1.0"
 dependencies = [
  "assert_matches",
  "futures 0.3.12",
+ "futures-timer 3.0.2",
  "lru",
  "maplit",
  "parity-scale-codec",
@@ -5166,6 +5167,7 @@ dependencies = [
  "rand 0.8.3",
  "sc-keystore",
  "sc-network",
+ "smallvec 1.6.1",
  "sp-application-crypto",
  "sp-core",
  "sp-keyring",