Fix backend test for environment without ChainerX
Fix backend test for environment without ChainerX

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