fix: another sealevel unit test flake (#4784)

Applies the same fix from
https://github.com/hyperlane-xyz/hyperlane-monorepo/pull/4655 to a flake
that showed up in this PRs CI:
https://github.com/hyperlane-xyz/hyperlane-monorepo/pull/4693#event-15004545304

rust/sealevel/programs/mailbox-test/src/functional.rs

@@ -1,3 +1,5 @@
+use std::thread::sleep;
+
 use borsh::BorshDeserialize;
 use hyperlane_core::{
     accumulator::incremental::IncrementalMerkle as MerkleTree, HyperlaneMessage, H256,
@@ -976,6 +978,10 @@ async fn test_process_errors_if_message_already_processed() {
     .await
     .unwrap();
 
+    // there's a race condition that isn't fixed by setting `CommitmentLevel::Confirmed`
+    // just wait a bit to ensure the message is processed
+    sleep(std::time::Duration::from_secs(1));
+
     let result = process(
         &mut banks_client,
         &payer,