Test backfill (#5109)

* Test backfill

* Revert cargo.toml

* Update beacon_node/beacon_chain/src/builder.rs

Co-authored-by: João Oliveira <hello@jxs.pt>

* Remove redundant code

beacon_node/beacon_chain/Cargo.toml

@@ -11,6 +11,7 @@ write_ssz_files = []  # Writes debugging .ssz files to /tmp during block process
 participation_metrics = []  # Exposes validator participation metrics to Prometheus.
 fork_from_env = [] # Initialise the harness chain spec from the FORK_NAME env variable
 portable = ["bls/supranational-portable"]
+test_backfill = []
 
 [dev-dependencies]
 maplit = { workspace = true }