By Alexandra Ulmer ATLANTA (Reuters) -Donald Trump’s campaign to play kingmaker in this year’s U.S. midterm elections faces its biggest test yet in Georgia on Tuesday as Republican voters decide whether to heed his demands to oust Governor Brian Kemp. Trump has backed former U.S. Senator David Perdue’s primary challenge against Kemp, who angered the former president by refusing to try to overturn his defeat in the 2020 election. While opinion polls show Kemp holding a wide lead over Perdue, another Trump-endorsed candidate, former football star Herschel Walker, is favored to snag the Republica…