Saves about 35% on total test time on my laptop.

Fixes #26471

Change-Id: I15b28b1bc00f889934d577dc7996864bbab10105
Reviewed-on: https://go-review.googlesource.com/c/153499
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>