math/big: gofmt nat_test.go

Was apparently checked in by https://go-review.googlesource.com/#/c/9253/
without being gofmt-ed.

TBR: agl

Change-Id: I4d011dbaa15b7c5e73ca71f724f32951a0302dae
Reviewed-on: https://go-review.googlesource.com/10353Reviewed-by: Robert Griesemer <gri@golang.org>