tests: base64 minor refactor for dryness
tests: base64 minor refactor for dryness

kevgeniy/coreutils,ebfe/coreutils,jbcrail/coreutils,keunwoo/coreutils,tempbottle/coreutils,Arcterus/coreutils,ebfe/coreutils,keunwoo/coreutils,tempbottle/coreutils,Heather/coreutils,kevgeniy/coreutils,wimh/coreutils,Arcterus/coreutils,uutils/coreutils,uutils/coreutils,Heather/coreutils,wimh/coreutils,jbcrail/coreutils,uutils/coreutils