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.

zerver/tests/test_management_commands.py

@@ -509,6 +509,7 @@ class TestExport(ZulipTestCase):
             content="Outbox emoji for export",
         )
         message = Message.objects.last()
+        assert message is not None
         do_add_reaction(
             self.example_user("iago"), message, "outbox", "1f4e4", Reaction.UNICODE_EMOJI
         )