Bugfix: ChainerMN testing
Bugfix: ChainerMN testing

This commit fixes a bug in the `get_device` in `chainermn.testing`,
which use native when specifying to use gpu with `gpu_id=0`,
as the `0` is considered as `False` in `if`.

pfnet/chainer,niboshi/chainer,chainer/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,hvy/chainer,hvy/chainer,niboshi/chainer,chainer/chainer,hvy/chainer,hvy/chainer,niboshi/chainer