Otherwise we end up with testp?.exe files after the tests run.

Updates #11058

Change-Id: Ieccfc42da6192622bdab1f9a411ccd50bb59fd5b
Reviewed-on: https://go-review.googlesource.com/68770
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>