math/big: test both bitLen and bitLen_g
math/big: test both bitLen and bitLen_g

Also: simpler, more direct test.

R=golang-dev, dave.andersen
CC=golang-dev
http://codereview.appspot.com/5573070

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