Transaction malleability in Testnet
What malleability means is that you can create a transaction that is identical in its meaning (same inputs, same outputs) but with a different TXID. That is problematic for 2nd layer protocols where transactions build onto each other but aren't necessarily published to chain.

But a "malleated" transaction is still a double spend. And it appears newer versions  of bitcoin-core that support RBF won't accept double spends into their mempool.

See code snippet here: Validation Error: Error #-26: 258: txn-mempool-conflict