tests: add a new test for testing changing or clearing topics test-topic.t is getting bigger and bigger. This patch introduces a new test which includes test for clearing and changing topics. Since the tests are moved to a new file, this patch, removes those tests from test-topic.t also.

# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)