Test large but not infinite arguments.

This CL adds a test which breaks s390x.  Don't submit until
a fix for that is figured out.

Update #26477

Change-Id: Ic86739fe3554e87d7f8e15482875c198fcf1d59c
Reviewed-on: https://go-review.googlesource.com/125641
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>