test: add test for gccgo-specific issue 8079

LGTM=gri
R=gri
CC=golang-codereviews, iant
https://codereview.appspot.com/159850044
math/big: Fixes issue 8920

(*Rat).SetString checks for denominator.

LGTM=gri
R=golang-codereviews, gri
CC=golang-codereviews
https://codereview.appspot.com/159760043

Committer: Robert Griesemer <gri@golang.org>