Refactor test directory structure to align Chainer's test dir

[kuenishi]

Addresses #155 .

[iwiwi]

Thanks for the PR!

I propose a rule the following rule, which is the same as Chainer: tests for chainermn/AAA/BBB is in tests/chainermn_tests/AA_tests/BBB_tests (What do you think about this rule?). If we adopt the above rule, I would like you to fix the following problems:

• Please move training_tests/extensions_tests to extensions_tests
• Please move test_link.py to links_tests (and rename if appropriate)

[kuenishi]

I think we have to follow the Chainer's test directory tree naming rule, that follows namespace, which is fine to me. Also we have to align ChianerMN's namespace to that of Chainer at some timing after 1.1 release. Especially multi node evaluator and checkpointer must be in chainermn.training.extensions , while it's now in chainermn.extensions as well as it's directory position.

[kuenishi]

So far I just pushed required change above.

[iwiwi]

Thanks. Previously, we decided not to follow the exact structure of namespaces (e.g., we don't think we need chainermn.training module) to keep our module/directory structures clean and concise. If you have opinions, let us discuss separately.

[kuenishi]

ah ok, I understand that decision. I won't do further work on this issue.