Add unit test for olsotest base class

Oslotest base class has special logic and uses environment variables
to trigger them.

Change-Id: Ic18b6fe528e1d95ad4390b9806aa34c050362690
Cleanup mock patches on BaseTestCase tearDown()

Instead of cleaning up any remaining mock patches in each unit test that
uses them, do it once on BaseTestCase tearDown().

Change-Id: I121ba554e16d83fc5908bb85afee4940f42dc4eb
Closes-Bug: 1323214