[release-branch.go1.8] testing: stop timeout-timer after running tests

Fixes #18845

Fixes #18870 (Go 1.8 backport)

Change-Id: Icdc3e2067807781e42f2ffc94d1824aed94d3713
Reviewed-on: https://go-review.googlesource.com/35956


Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 7d8bfdde)
Reviewed-on: https://go-review.googlesource.com/36125