Fix test discovery by new testtools

New testtools finds tests in tests/__init__.py and attempts to execute
them. Stop this from happening by not inheriting from the base test
class in __init__.py. Instead mix in the base test class where we
actually need to run the tests in test_allocator.py

Change-Id: I9733ac407d415c90e7dcaa2c6e6a972e75ce6d42