Don't test so much at once.

Fixes #10278

Change-Id: I32a9cb81a3cffecc7ce4f83c35a4b589bcd3a9f7
Reviewed-on: https://go-review.googlesource.com/8213Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>