To make it overcomplicated for git just to fake a point doesn't do it for me.
I am yet to see a real value to use JJ. Those kind of built-up posts I am seeing about JJ on HN tend to push me in the opposite direction.
1) do your test in your current branch, it's related to what you are doing. 2) commit, co master, co -b newtestbranch, PR it, merge in your branch after. You're going to squash rebase at the end anyway...