tests: Use assertion to enforce None-checks in tests.

This fixes a batch of mypy errors of the following format:
'Item "None" of "Optional[Something]" has no attribute "abc"

Since we have already been recklessly using these attritbutes
in the tests, adding assertions beforehand is justified presuming
that they oughtn't to be None.

analytics/tests/test_counts.py

@@ -240,6 +240,7 @@ class TestProcessCountStat(AnalyticsTestCase):
         self, stat: CountStat, end_time: datetime, state: int = FillState.DONE
     ) -> None:
         fill_state = FillState.objects.filter(property=stat.property).first()
+        assert fill_state is not None
         self.assertEqual(fill_state.end_time, end_time)
         self.assertEqual(fill_state.state, state)