You must run bitcoind or bitcoin-qt with --testnet (testnet3) or --testnet4 arg to launch it in testnet.
I'm assuming the next version of Bitcoin will run v4 with the --testnet command(?)
There's no ETA but it can't be the next major release since the PR containing the change is still in "
draft" status.
As of now, I can't see it in v30.0 and v31.0's milestones.
You can bookmark this to know when it'll be released:
https://github.com/bitcoin/bitcoin/pull/31974And here's where the discussions are:
https://github.com/bitcoin/bitcoin/issues/31975