Reorganize test module structure

Moved test modules:
 tests/             ->  tests/unit
 test_ci/           ->  tests/ci
 tests_functional   ->  tests/functional
 rally/hacking      ->  tests/hacking

Add testing read me file

Co-Authored-By: Boris Pavlovic <boris@pavlovic.me>
Co-Authored-By: Andrey Kurilin <akurilin@mirantis.com>

Change-Id: I57c09d892da4adf863c358a4d63e3543b50d10b7