bot tests: Adjust TestCase inheritance to avoid need to skip/filter.

Previously the test-bots script filtered out base-class tests from
BotTestCase. With this change, BotTestCase continues to inherit from
unittest.TestCase, but the default test_* methods previously in this
class are now in a new DefaultTests class, which does not. Instead, each
bot needs to inherit from BotTestCase and DefaultTests *explicitly*.

This avoids the need to filter out the base-class tests, which
simplifies the test-bots script, and may ease any migration to eg.
pytest.

The DefaultTests class does require some non-implemented methods which
BotTestCase provides.

zulip_bots/zulip_bots/test_lib.py

@@ -88,6 +88,29 @@ class StubBotHandler:
             raise Exception('The bot is giving too many responses for some reason.')
 
 
+class DefaultTests:
+    bot_name = ''
+
+    def make_request_message(self, content: str) -> Dict[str, Any]:
+        raise NotImplementedError()
+
+    def get_response(self, message: Dict[str, Any]) -> Dict[str, Any]:
+        raise NotImplementedError()
+
+    def test_bot_usage(self) -> None:
+        bot = get_bot_message_handler(self.bot_name)
+        assert bot.usage() != ''
+
+    def test_bot_responds_to_empty_message(self) -> None:
+        message = self.make_request_message('')
+
+        # get_response will fail if we don't respond at all
+        response = self.get_response(message)
+
+        # we also want a non-blank response
+        assert len(response['content']) >= 1
+
+
 class BotTestCase(unittest.TestCase):
     bot_name = ''
 
@@ -154,19 +177,6 @@ class BotTestCase(unittest.TestCase):
         bot_class = type(get_bot_message_handler(self.bot_name))
         bot_class.validate_config(config_data)
 
-    def test_bot_usage(self) -> None:
-        bot = get_bot_message_handler(self.bot_name)
-        self.assertNotEqual(bot.usage(), '')
-
-    def test_bot_responds_to_empty_message(self) -> None:
-        message = self.make_request_message('')
-
-        # get_response will fail if we don't respond at all
-        response = self.get_response(message)
-
-        # we also want a non-blank response
-        self.assertTrue(len(response['content']) >= 1)
-
     def mock_http_conversation(self, test_name: str) -> Any:
         assert test_name is not None
         http_data = read_bot_fixture_data(self.bot_name, test_name)