tests: Remove use of 'oslo_db.sqlalchemy.test_base'
tests: Remove use of 'oslo_db.sqlalchemy.test_base'

The various test cases and mixins in 'oslo_db.sqlalchemy.test_base' have
been deprecated since oslo.db 4.34.0 (March 2018). Remove use of these.

Change-Id: Ic4ba41e61de3a33d6779b53bb0274e5e115e335a

openstack/murano,openstack/murano