Enable check_double_backward tests for ChainerX arrays
Enable check_double_backward tests for ChainerX arrays

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