Force population of _result_cache by adding test for valid len

Previously, [this test](https://github.com/cbmi/avocado/blob/2.x/tests/cases/models/tests.py#L32-L33) in the DataField cache test would fail because the _result_cache is not populated since filter does not set that property of the QuerySet object. It is not set until a
method is called that forces it to be set so by calling [len](https://github.com/django/django/blob/master/django/db/models/query.py#L89-L92) we will force _result_cache to be set and also allow us to test and make sure we got something back from our filter call.