Unit tests for scrubber

This patch adds a place for unit tests for the scrubber. The test in
it addresses the change made at:
https://review.openstack.org/#/c/29405/

addresses bug: 1195423
blueprint: refactoring-better-faster-stronger-functional-tests

Change-Id: I02da0847c3f93474f478c8740e4649280a02eae1
Merge "Unit tests for scrubber"