tests/common: more communicative assertion failure messages
tests/common: more communicative assertion failure messages

often when stdout is wrong, differences in stderr are
more useful for determining the cause

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