Allow starting testnet from JSON state

beacon_node/src/config.rs

@@ -186,6 +186,7 @@ fn process_testnet_subcommand(
             let start_method = match format {
                 "yaml" => BeaconChainStartMethod::Yaml { file },
                 "ssz" => BeaconChainStartMethod::Ssz { file },
+                "json" => BeaconChainStartMethod::Json { file },
                 other => return Err(format!("Unknown genesis file format: {}", other)),
             };