Move tests back to neutronclient package

Neutron itself has all of its tests in neutron/tests, as do 2/3 of
the projects in OpenStack. There are a wealth of reasons to put
them in the package, not the least of which that tests/__init__.py
implies a code living in a global namespace called tests.

Change-Id: I2aaacfe59d6790648aa4587e89e6dd28a9149a6c