Use oslotest by default

oslotest has been added to the requirements for new projects
created by oslo-cookiecutter, but the default test file still uses
a local copy of the base file.  Let's just use oslotest instead.

Also removes an unnecessary license header from the blank
__init__.py in the tests directory.

Change-Id: If96ae2d8fec05f6b0e86c08b0451567665ce80f4
Merge "Use oslotest by default"