net/rpc: skip TestGobError on Plan 9
net/rpc: skip TestGobError on Plan 9

LGTM=bradfitz
R=rsc, bradfitz
CC=aram, golang-codereviews
https://codereview.appspot.com/154140043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go