add tests for Variable.xp
add tests for Variable.xp

keisuke-umezawa/chainer,jnishi/chainer,wkentaro/chainer,niboshi/chainer,tkerola/chainer,ktnyt/chainer,ktnyt/chainer,keisuke-umezawa/chainer,niboshi/chainer,hvy/chainer,jnishi/chainer,okuta/chainer,chainer/chainer,wkentaro/chainer,wkentaro/chainer,jnishi/chainer,hvy/chainer,okuta/chainer,chainer/chainer,hvy/chainer,niboshi/chainer,jnishi/chainer,chainer/chainer,keisuke-umezawa/chainer,okuta/chainer,okuta/chainer,rezoo/chainer,ronekko/chainer,chainer/chainer,keisuke-umezawa/chainer,ktnyt/chainer,hvy/chainer,ktnyt/chainer,wkentaro/chainer,niboshi/chainer,anaruse/chainer,pfnet/chainer