go.tools/cmd/godoc: prevent duplicate GOROOT/GOPATH in environment in TestTypeAnalysis
go.tools/cmd/godoc: prevent duplicate GOROOT/GOPATH in environment in TestTypeAnalysis

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/94430044

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