This makes the test easier to run in isolation and easier to change,
and simplifies the code to run the tests in parallel.

updates #13467

Change-Id: I5622b5cc98276970347da18e95d071dbca3c5cc1
Reviewed-on: https://go-review.googlesource.com/63276
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>