math/big: make tests faster by reducing coverage in --test.short mode.
math/big: make tests faster by reducing coverage in --test.short mode.

The time to test all of math/big is now:
 default      => ~3min
 --test.short => 150ms

R=rsc
CC=golang-dev
https://codereview.appspot.com/7223054
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go