Unless the leaderboard is live, you can't really test it. Which means publishing.
While you can publish a game, you can also prevent it from hitting the store in it's current form by choosing manual release. Once the leaderboard is live, Gamecenter will log you in as normal.
Another work around I thought of but never tested was taking an already live leaderboard and referencing it back to the test app. I don't think it'll work unless you line up the app com name to match what the live version has, but technically, you could use another working game and just use the testflight versions and never publish it.
Thanks for your input — - I actually have also created a bug report on the Construct 3 Github page, and it looks like they are in the process of revamping their Game Center and IAP plugins. I will have a look at your solution but I think I will also wait for their release!
Thanks