testing/provision: added method to add a fake unit.

related to #806.
testing: fixed AddUnit behaviour.

AddUnit updates the app.unitLen value.

related to #806.
app: fixed app tests to works with provision.Unit.

* use app.Units() instead app.Unit.
* use AddUnit and Provision to add units instead store it on database.
* clean queue messages on teardown.

related to #806.