I didn't build a test garden per se, but I did have my wife bury in the backyard at different depths and different locations, the various denominations of our Canadian coins.
I then spent a few weekends working out what was good and what was junk.
Eventually I found all but one coin and had to get her to show me where it was. She was unaware that she buried the coin right overtop of a piece of siding, so when I got a giant hit when my detector was well over a foot off the ground........well I didn't dig it.
I still don't know if I would have been able to figure it out now if I'm honest, but there is certainly some merit to practicing in your own yard first.
The other reason why I would suggest practicing in your own yard first, is so that you can learn how to dig a proper plug.
You may as well screw up your own grass to start, as only your wife/husband/partner will give you crap, instead of a groundskeeper or a law official.
There are a zillion threads on how to dig a plug, or YouTube videos, so I would take the time and watch them too.
Good luck with your new machine.