Fix clippy errors on tests (#2160)

## Issue Addressed

There are some clippy error on tests.


## Proposed Changes

Enable clippy check on tests and fix the errors. 💪

beacon_node/beacon_chain/src/validator_pubkey_cache.rs

@@ -238,6 +238,7 @@ mod test {
         builder.build()
     }
 
+    #[allow(clippy::needless_range_loop)]
     fn check_cache_get(cache: &ValidatorPubkeyCache, keypairs: &[Keypair]) {
         let validator_count = keypairs.len();