Tests exist that call m.Run in a loop‽
Now we have one too.

Fixes #23129.

Change-Id: I8cbecb724f239ae14ad45d75e67d12c80e41c994
Reviewed-on: https://go-review.googlesource.com/83956
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>