engine: Add dao_unit_test to DeleteCompletedJobsOlderThanDate

Added a unit test to check that DeleteCompletedJobsOlderThanDate deletes
jobs and steps that have no async-tasks

Change-Id: Iba883b90fc8fc89096a98beec796d4600714eb15
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>