go.tools/cmd/vet: disable TestVet on Plan 9
go.tools/cmd/vet: disable TestVet on Plan 9

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews
https://golang.org/cl/64410043

cw2018/tools,argandas/tools,maddyonline/tools,cmars/tools,nicot/tools,runcom/tools,josharian/tools,golang/tools,sshetty10/tools,4ad/tools,argandas/tools,golang/tools,glasser/golang-tools,springning/tools,springning/tools,jdhenke/tools,4ad/tools,cmars/tools,jdhenke/tools,cw2018/tools,syreclabs/go-tools,golang/tools,jdhenke/tools,maddyonline/tools,golang/tools,cw2018/tools,matryer/tools,nicot/tools,jdhenke/tools,nicot/tools,cmars/tools,nicot/tools,dmitris/tools,glasser/golang-tools,matryer/tools,maddyonline/tools,argandas/tools,4ad/tools,sshetty10/tools,dmitris/tools,matryer/tools,magiccao/tools,maddyonline/tools,runcom/tools,syreclabs/go-tools,cw2018/tools,glasser/golang-tools,magiccao/tools,josharian/tools,syreclabs/go-tools,runcom/tools,springning/tools,argandas/tools,dmitris/tools,cmars/tools,matryer/tools,sshetty10/tools,glasser/golang-tools,magiccao/tools,josharian/tools,golang/tools,syreclabs/go-tools,sshetty10/tools,josharian/tools,runcom/tools,springning/tools,dmitris/tools,syreclabs/go-tools,syreclabs/go-tools,magiccao/tools