Generalize base test case into common code

Code in BaseTestCase from openstack/common/test.py and
test/utils.py look similar. To reduce code duplication, base test case
generalized to common.test. Thus it can be reused in other projects.

Co-Authored-By: Ilya Pekelny <ipekelny@mirantis.com>
Change-Id: I60b6b41d2e6cd379616acec92242740b4bb00da1
Merge "Generalize base test case into common code"