Add a short wait at the beginning of randomized workload (#20667)

## Description 

DKG may take seconds to complete, which cause randomness transaction
experiencing `TooOldTransactionPendingOnObject`, which has 500ms queue
age limit.

Thanks @aschran for the insight!

## Test plan 

How did you test the new or updated feature?

---

## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

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that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] Indexer: 
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] REST API:

crates/sui-benchmark/src/workloads/randomized_transaction.rs

@@ -14,6 +14,7 @@ use async_trait::async_trait;
 use futures::future::join_all;
 use rand::Rng;
 use std::sync::Arc;
+use std::time::Duration;
 use sui_test_transaction_builder::TestTransactionBuilder;
 use sui_types::base_types::{ObjectID, ObjectRef, SequenceNumber, SuiAddress};
 use sui_types::crypto::{get_key_pair, AccountKeyPair};
@@ -406,6 +407,10 @@ impl Workload<dyn Payload> for RandomizedTransactionWorkload {
         proxy: Arc<dyn ValidatorProxy + Sync + Send>,
         system_state_observer: Arc<SystemStateObserver>,
     ) {
+        // We observed that randomness may need a few seconds until DKG completion, and this may
+        // causing randomness transaction fail with `TooOldTransactionPendingOnObject` error.
+        // Therefore, wait a few seconds at the beginning to give DKG some time.
+        tokio::time::sleep(Duration::from_secs(5)).await;
         if self.basics_package_id.is_some() {
             return;
         }