Adding my vote to not only allow setting back to untested, but also provide additional statuses to customize. Sometimes you do make a mistake and just need to set the case back to untested.
In my case, I previously re-purposed ‘untested’ to represent more of a default ‘new’ case due to the way it reports (in our world there is a difference between new and untested). Now, I have used all available statuses and have a new critical one that needs to be added so I have to sacrifice an existing one. To do this I am trying to move a batch from a current status over to the untested status before I re-purpose it. (FYI, this is another missing feature, the ability for a master admin to batch move tickets from one status to another at the project level). Since you can’t set cases back to ‘untested’, I am not able to do this and now have no way to make this status field change successfully.