math/big: add test for recursive division panic

The vulnerability that allowed this panic is
CVE-2020-28362 and has been fixed in a security
release, per #42552.

Change-Id: I774bcda2cc83cdd5a273d21c8d9f4b53fa17c88f
Reviewed-on: https://go-review.googlesource.com/c/go/+/277959


Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>