go.tools/oracle: fix TestOracle on Plan 9
go.tools/oracle: fix TestOracle on Plan 9

LGTM=minux.ma
R=golang-codereviews, lucio.dere, gobot, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/64400043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old