Sync wrong dbg assertion (#2821)

## Issue Addressed

Running a beacon node I triggered a sync debug panic. And so finally the time to create tests for sync arrived. Fortunately, te bug was not in the sync algorithm itself but a wrong assertion

## Proposed Changes

- Split Range's impl from the BeaconChain via a trait. This is needed for testing. The TestingRig/Harness is way bigger than needed and does not provide the modification functionalities that are needed to test sync. I find this simpler, tho some could disagree.
- Add a regression test for sync that fails before the changes.
- Fix the wrong assertion.

beacon_node/network/src/status.rs

@@ -1,3 +1,5 @@
+use std::sync::Arc;
+
 use beacon_chain::{BeaconChain, BeaconChainError, BeaconChainTypes};
 
 use lighthouse_network::rpc::StatusMessage;
@@ -23,3 +25,9 @@ impl<T: BeaconChainTypes> ToStatusMessage for BeaconChain<T> {
         })
     }
 }
+
+impl<T: BeaconChainTypes> ToStatusMessage for Arc<BeaconChain<T>> {
+    fn status_message(&self) -> Result<StatusMessage, BeaconChainError> {
+        <BeaconChain<T> as ToStatusMessage>::status_message(self)
+    }
+}