Add unittest for Variable.unchain_backward
Add unittest for Variable.unchain_backward

jnishi/chainer,cemoody/chainer,chainer/chainer,muupan/chainer,ikasumi/chainer,niboshi/chainer,tereka114/chainer,ytoyama/yans_chainer_hackathon,laysakura/chainer,ktnyt/chainer,okuta/chainer,delta2323/chainer,Kaisuke5/chainer,1986ks/chainer,benob/chainer,hvy/chainer,t-abe/chainer,tscohen/chainer,t-abe/chainer,wavelets/chainer,wkentaro/chainer,wkentaro/chainer,kashif/chainer,benob/chainer,truongdq/chainer,wkentaro/chainer,rezoo/chainer,ysekky/chainer,keisuke-umezawa/chainer,masia02/chainer,jnishi/chainer,niboshi/chainer,okuta/chainer,ktnyt/chainer,woodshop/complex-chainer,yanweifu/chainer,woodshop/chainer,ktnyt/chainer,sinhrks/chainer,pfnet/chainer,keisuke-umezawa/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,sou81821/chainer,umitanuki/chainer,niboshi/chainer,jnishi/chainer,keisuke-umezawa/chainer,cupy/cupy,elviswf/chainer,aonotas/chainer,sinhrks/chainer,jnishi/chainer,bayerj/chainer,truongdq/chainer,kikusu/chainer,ktnyt/chainer,cupy/cupy,hvy/chainer,hidenori-t/chainer,kikusu/chainer,keisuke-umezawa/chainer,ronekko/chainer,jfsantos/chainer,chainer/chainer,kiyukuta/chainer,kuwa32/chainer,minhpqn/chainer,AlpacaDB/chainer,chainer/chainer,muupan/chainer,tkerola/chainer,okuta/chainer,okuta/chainer,anaruse/chainer,chainer/chainer,cupy/cupy,wkentaro/chainer,AlpacaDB/chainer,tigerneil/chainer,cupy/cupy