testing: improve diagnosis of a potential misuse of RunParallel
testing: improve diagnosis of a potential misuse of RunParallel

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/68230045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang