gossip-support: add unittests for update authorities (#3258)

~The previous fix was actually incomplete because we update the
authorties only on the situation where we decided to reconnect because
we had a low connectivity issue. Now the problem is that
update_authority_ids use the list of connected peers, so on restart that
does contain anything, so calling immediately after
issue_connection_request won't detect all authorities, so we need to
also check every block as the comment said, but that did not match the
code.~

Actually the fix was correct the flow is follow if more than 1/3 of the
authorities can not be resolved we set last_failure and call
`ConnectToResolvedValidators`.

We will call UpdateAuthorities for all the authorities already connected
and for which we already know the address and for the ones that will
connect later on `PeerConnected` will have the AuthorityId field set,
because it is already known, so approval-distribution will update its
cache topology.

---------

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>
Co-authored-by: Alexander Samusev <41779041+alvicsam@users.noreply.github.com>

polkadot/node/network/gossip-support/src/lib.rs

@@ -63,8 +63,12 @@ use metrics::Metrics;
 const LOG_TARGET: &str = "parachain::gossip-support";
 // How much time should we wait to reissue a connection request
 // since the last authority discovery resolution failure.
+#[cfg(not(test))]
 const BACKOFF_DURATION: Duration = Duration::from_secs(5);
 
+#[cfg(test)]
+const BACKOFF_DURATION: Duration = Duration::from_millis(500);
+
 /// Duration after which we consider low connectivity a problem.
 ///
 /// Especially at startup low connectivity is expected (authority discovery cache needs to be
@@ -271,8 +275,8 @@ where
 					)
 					.await?;
 				}
-				// authority_discovery is just a cache so let's try every leaf to detect if there
-				// are new authorities there.
+				// authority_discovery is just a cache so let's try every time we try to re-connect
+				// if new authorities are present.
 				self.update_authority_ids(sender, session_info.discovery_keys).await;
 			}
 		}