Test CostCenter.get_sales() using a set

On rare ocasions, 'test_costcente' will fail at:
  self.assertEquals(list(cost_center1.get_sales()), [sale1, sale2])

That because sometimes get_sales come as [sale2, sale1] instead of
[sale1, sale2]. So, check using a set that compares the content, and not the
positions.

Change-Id: Id3c0c9d5ac407aa04200059fc820c5b2eb97de0c
Merge "Test CostCenter.get_sales() using a set"