Revert "rolls back chained Merkle shreds for testnet downgrade (#3194)…

…" (#3503)

This reverts commit 69916f1.

(cherry picked from commit db4f2d6)

turbine/src/broadcast_stage/standard_broadcast_run.rs

@@ -511,7 +511,7 @@ fn should_chain_merkle_shreds(_slot: Slot, cluster_type: ClusterType) -> bool {
         ClusterType::Development => true,
         ClusterType::Devnet => false,
         ClusterType::MainnetBeta => false,
-        ClusterType::Testnet => false,
+        ClusterType::Testnet => true,
     }
 }