More tests for new request based statement distribution (#2875)

* More test coverage.

* Preserve peer order.

* Better test coverage.

* Even more test coverage.

* Add doc comment to `IndexMap`.

* Fix flaky test.

* Review remarks.

* Review remarks.

polkadot/node/network/statement-distribution/src/requester.rs

@@ -178,6 +178,11 @@ pub async fn fetch(
 		// All our peers failed us - try getting new ones before trying again:
 		match try_get_new_peers(relay_parent, candidate_hash, &mut sender, &span).await {
 			Ok(Some(mut peers)) => {
+				tracing::trace!(
+					target: LOG_TARGET,
+					?peers,
+					"Received new peers."
+				);
 				// New arrivals will be tried first:
 				new_peers.append(&mut peers);
 			}