go.tools/cmd/stringer: add .exe suffix to executable name in TestEndToEnd
go.tools/cmd/stringer: add .exe suffix to executable name in TestEndToEnd

Fixes windows build

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/140360043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools