but it's easier to just wait for testnet3 to be shredded and replaced by the new version by default
New version released:
https://groups.google.com/g/bitcoindev/c/EmAOO-NbmzwIt is release candidate, but it contains fully working testnet4 implementation.
Of course, for CPU-based mining, some modifications are needed, but I think grinding coins with bitcoin-util should still be possible. I will think about some kind of such implementation.