Fix importing errors in unit tests

[keisukefukuda]

Some tests lack import chainer.testing, so the produce the following error when invoked alone by pytest.

./tests/chainermn_tests/functions_tests/test_point_to_point_communication.py
============================= test session starts ==============================
platform linux -- Python 3.6.0, pytest-3.0.5, py-1.4.32, pluggy-0.4.0 -- /home/kfukuda/.pyenv/versions/anaconda3-4.3.1/bin/python
cachedir: .cache
rootdir: /home/kfukuda/chainermn, inifile:
collecting ... collected 0 items / 1 errors

==================================== ERRORS ====================================
 ERROR collecting tests/chainermn_tests/functions_tests/test_point_to_point_communication.py
tests/chainermn_tests/functions_tests/test_point_to_point_communication.py:170: in <module>
    @chainer.testing.attr.gpu
E   AttributeError: module 'chainer' has no attribute 'testing'
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!!
=========================== 1 error in 1.42 seconds ============================

This PR fixes these issues simply by adding import chainer.testing .