Bitcoin Core. By the way: you don't need to "convert" anything, you can instead create a transaction, sending coins to the mainnet address, and then sign that transaction under testnet. Also, if you want to move those coins, then again: you can create a transaction under testnet, and sign it from your mainnet node. In this way, no conversions are needed.

Another benefit of doing that, is testing offline signing, which is a good habit in general.