Fix structure of unit tests in oslo.messaging (part 1)

Even in case of libraries and when they are not big like nova.
It is better to use next rules:

1) structure of tests directory is the same as a root of project
Tests directory will be well organized, and it will be simple to find
where to write tests

2) names for end modules should be test_<name_of_testing_module>.py

Change-Id: Ie13d1b709e863f6a9babaaa2bf1e042ef9251671