Fix use of conf_fixture

As the name implies, conf_fixture.ConfFixture is a fixture. It should
be used by calling self.useFixture() instead of calling it directly.
This ensures that the setUp() method is called correctly.

If the test classes are executed individually, the database config
options are not registered correctly and could result in this failure
for each test case:

NoSuchOptError: no such option in group database: backend

Closes-bug: 1396703

Change-Id: Iaf7e1aedaacf24508e55f4d6975946f83ef833a6
Merge "Fix use of conf_fixture"