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/sync/range_sync/mod.rs

@@ -2,6 +2,7 @@
 //! peers.
 
 mod batch;
+mod block_storage;
 mod chain;
 mod chain_collection;
 mod range;