sum: fix sysv_stdin testcase
sum: fix sysv_stdin testcase

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