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 }

beacon_node/beacon_chain/src/builder.rs

@@ -846,10 +846,14 @@ where
         let genesis_backfill_slot = if self.chain_config.genesis_backfill {
             Slot::new(0)
         } else {
-            let backfill_epoch_range = (self.spec.min_validator_withdrawability_delay
-                + self.spec.churn_limit_quotient)
-                .as_u64()
-                / 2;
+            let backfill_epoch_range = if cfg!(feature = "test_backfill") {
+                3
+            } else {
+                (self.spec.min_validator_withdrawability_delay + self.spec.churn_limit_quotient)
+                    .as_u64()
+                    / 2
+            };
+
             match slot_clock.now() {
                 Some(current_slot) => {
                     let genesis_backfill_epoch = current_slot