Testing unattended-upgrades with actual package updates
Slightly different from what you are asking, but should accomplish the same thing:
Add a PPA that has a newer version of an installed package:
sudo apt install certbot
sudo add-apt-repository ppa:certbot/certbot
See if unattended-upgrades updates the package.
I haven't tested this, so it's possible there is something about certbot that makes it not a good PPA to use. Others to try might include nginx and the PPA from ppa:nginx/stable.