Add xchainer::NativeDevice thread safety test
Add xchainer::NativeDevice thread safety test

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