Look, we realize that you can’t really test for abandoning Windows Vista and Windows XP support for Diablo III patch 2.6.1. But that change is coming for this patch, and it’s noted right at the start of the patch notes. The patch is also bringing a huge pile of new item changes, including changed drop rates, changed set effects, and changed abilities on individual items. If you have a spreadsheet for comparing the values of your various class armor sets, said spreadsheet will get more complicated on the test realm.
A few ability changes are also coming with the patch, most notably for the Necromancer, and players can purchase legendary set bags for Blood Shards on the test realm only. Further changes are possible and even likely, so more testing is probably a good idea; just make sure you’re not testing on a computer still running Windows XP.