modifying test about connection

[Davidrjx]

new test package called connections and move related modules in;
new related test methods;

[Davidrjx]

do someone as an assignee make the review for this request?

[wojcikstefan]

Hi @Davidrjx ! Could you summarize these changes in the PR's description? Just a few brief sentences what it gives us, why it's useful, whether it's backward compatible, what was the problem with the previous structure, etc.

Ping me please when that's done and I'll review.

[Davidrjx]

@wojcikstefan thanks for your reply. The pull request mainly contains:
(1) for replicaset and readpreference option in mongodb, do some adjustments in mongoengine connection module to make connection setting more suitable for Pymongo client.
previous possible problems: a bit unclear/incorrect connection setting arguments processing in kwargs/uri_opt.
(2) at the same time, combined with modifications above, also i do the same thing for connection test, which involves test module location tuning and methods new addition.
previous possible problems: uncomfortable module archive and incomplete tests.
(3) above tests are run with Py2(2.7),Py3(3.4) and Pymongo2(2.9&2.8),Pymongo3(3.3)

[Davidrjx]

@wojcikstefan please review