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

dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,tv42/old-go,gnanderson/go,tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go