Improved SubqueryTests to cover some potential problem areas for database backends that must do some trickery for slicing

subquerytests.py

class SubqueryTests(TestCase):
    def setUp(self):
        DumbCategory.objects.create(id=1)
        DumbCategory.objects.create(id=2)
        DumbCategory.objects.create(id=3)
        DumbCategory.objects.create(id=4)

    def test_ordered_subselect(self):
        "Subselects honor any manual ordering"
        try:
            query = DumbCategory.objects.filter(id__in=DumbCategory.objects.order_by('-id')[0:2])
            self.assertEqual(set(query.values_list('id', flat=True)), set([3,4]))

            query = DumbCategory.objects.filter(id__in=DumbCategory.objects.order_by('-id')[:2])
            self.assertEqual(set(query.values_list('id', flat=True)), set([3,4]))

            query = DumbCategory.objects.filter(id__in=DumbCategory.objects.order_by('-id')[1:2])
            self.assertEqual(set(query.values_list('id', flat=True)), set([3]))

            query = DumbCategory.objects.filter(id__in=DumbCategory.objects.order_by('-id')[2:])
            self.assertEqual(set(query.values_list('id', flat=True)), set([1,2]))
        except DatabaseError as e:
            # Oracle and MySQL both have problems with sliced subselects.
            # This prevents us from even evaluating this test case at all.
            # Refs #10099
            self.assertFalse(connections[DEFAULT_DB_ALIAS].features.allow_sliced_subqueries, str(e))

    def test_sliced_delete(self):
        "Delete queries can safely contain sliced subqueries"
        try:
            DumbCategory.objects.filter(id__in=DumbCategory.objects.order_by('-id')[0:1]).delete()
            self.assertEqual(set(DumbCategory.objects.values_list('id', flat=True)), set([1,2,3]))

            DumbCategory.objects.filter(id__in=DumbCategory.objects.order_by('-id')[1:2]).delete()
            self.assertEqual(set(DumbCategory.objects.values_list('id', flat=True)), set([1,3]))

            DumbCategory.objects.filter(id__in=DumbCategory.objects.order_by('-id')[1:]).delete()
            self.assertEqual(set(DumbCategory.objects.values_list('id', flat=True)), set([3]))
        except DatabaseError as e:
            # Oracle and MySQL both have problems with sliced subselects.
            # This prevents us from even evaluating this test case at all.
            # Refs #10099
            self.assertFalse(connections[DEFAULT_DB_ALIAS].features.allow_sliced_subqueries, str(e))