The context tests were flaky under Go 1.6 on Windows. Make them
slower, but make up for the slowness by parallelizing them.

These tests don't run on Go 1.7+ in the subrepo. They were deflaked in
the standard library's context in Go 1.7.

Updates golang/go#11811

Change-Id: I8dc8d9e13e72e87b4444e92d2316dd95bd7d066d
Reviewed-on: https://go-review.googlesource.com/34288
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>