Ah, you talking about that place filled with Clogon-build clones, Guidedshot campers and Blinkers? No worries, the testers aren't having much of your so-called "advantage". In fact, the arena will be more balanced in a way that it will be easier for everyone to beat these wryly common builds I mentioned above.
Only active testers who ever made it to the top in eternal, if I recall correctly, are me and Shay though I'm not there anymore because it's not built for AI use and I got bored and I'm not participating in arena matches anymore. I'm not even using stuff from the new patch lol (except a certain eternal item which got a changed vanilla affix)
Also, many of the testers are casuals, so it's not like they can get the new legends found at the highest floors and highest difficulties even if they wanted to.