Fix structure of unit tests in oslo.messaging (part 3 last)

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: I75b4b3df4fffc8dfe389c547ae7a004d1b278ecc
Merge "Fix structure of unit tests in oslo.messaging (part 3 last)"