Use the cli to set the joomla update channel
You have FOUR choices as shown in the error message when you get it wrong
[default, next, testing, custom]

If you try to change the joomla update channel to any of those 4 choices then you will get a success message and when you use the cli to check which channel you are on it will confirm your choice

However there are only three possible update channels now since the testing channel was removed #43147 from the web interface but the code is still present in the console libraries\src\Console\CheckJoomlaUpdatesCommand.php

You should not be able to successfully set the update channel to testing from the CLI but it appears that you can