Add .exe to exectable name, so it can be executed on windows.
Use proper windows paths when searching vet output.
Replace Skip with Skipf.

Fixes build

Change-Id: Ife40d8f5ab9d7093ca61c50683a358d4d6a3ba34
Reviewed-on: https://go-review.googlesource.com/10742
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Patrick Mézard <patrick@mezard.eu>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>