This is to support https://golang.org/cl/18057, which is going to add
Windows support to this directory.  Better to write the test in Go then
to have both test.bash and test.bat.

Update #13494.

Change-Id: I4af7004416309e885049ee60b9470926282f210d
Reviewed-on: https://go-review.googlesource.com/20892
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>