Right now it is always pkgname.test.exe, but if pkgname is
patch or install or setup or update, Windows thinks that
running it will install new software, so it pops up a dialog
box asking for more permission.
Renaming the binary avoids the Windows security check.

This only applies to the binary that the Go command writes
to its temporary work directory. If the user runs 'go test -c'
or any of the other ways to generate a test binary, it will
continue to use pkgname.test.exe.

Fixes #8711.

LGTM=bradfitz
R=golang-codereviews, r
CC=alex.brainman, bradfitz, golang-codereviews, iant
https://golang.org/cl/146580043