Mining on testnet is already a losers game. The reward is too low, and you need an ASIC if you don't want to wait for the 20 minute difficulty reset to 1. And even then if you get lucky you only are rewarded 0.012tBTC plus fees.

You'll spend more than 4 days with an S19 to mine a single tBTC not to mention the energy cost at 3,250 watts.  It's MUCH cheaper to buy testnet coins. Testnet coins would have to be worth around $30/40 per coin to make mining on testnet worth the energy and hardware costs.

Here's a bitcoin testnet mining calculator: https://buytestnet.com/bitcoin-testnet-mining-calculator.php