For a double spend, on
-testnet create a transaction and let it broadcast. Restart bitcoind or bitcoin-qt with
-tesnet -zapwallettxes=2 and then go ahead and respend your utxo’s.
Note: I’ll be honest, I haven’t actually any BTC on testnet to try this with, you may also have to delete your testnet
mempool.dat before restarting the node – try it?