ML2: Add tests to validate quota usage tracking

ML2: Add tests to validate quota usage tracking

Ensure that event handlers are invoked upon completion of
ML2 operations which add or remove tracked resources.
Also validate that the event handlers are called for the
appropriate resources and that quota usage's dirty bit
is set and unset as expected.

These are not unit tests, but added in the unit test tree
as they leverage code both from the DB unit test and the ML2
unit test framework. This module has indeed been added to
the 'exclusion list' in check_unit_test_structure.sh, and
should be moved to the functional test tree together with
the other modules.

Closes-Bug: #1499358

Change-Id: I78c432c35f3f3339607cd533019ae6d0fa2a5cd6