All testnet magic bytes / versions can be set at once with a boolean flag
--testnet
Or versions can be set custom individually with a string or hex option
--xprv-version 'xprv|tprv|mainnet|testnet|hex'
--xpub-version 'xpub|tpub|mainnet|testnet|hex'
--wif-version 'mainnet|testnet|hex'
--addr-version 'mainnet|testnet|hex'