Prepare for unit test reorg

Prepare for unit test reorg

The unit test reorg is about moving files around so a test module is
clearly associated with the code module it targets, but the test
modules in this change needed to be manually merged because they both
targeted the same module.

test_api_v2 is also updated to use the path of neutron/tests/base.py
as the root of path to test implementations of extensions.

Change-Id: I432b84339e51c26ef0aa26d44e29b5a3311626ad
Implements: bp/reorganize-unit-test-tree