embedded bots: Add tests for verification of embedded bot services.

Add test to check if the embedded bot service being used is in the
registry or not.
Add test to check if the bot being added to the registry has a valid
bot corresponding to it.
Move 'get_bot_handler' to 'zerver/lib/bot_lib.py' as it is an independent
function, not related to the 'EmbeddedBotWorker' class that it was
previously a part of.

zerver/worker/queue_processors.py

@@ -41,7 +41,7 @@ from zerver.context_processors import common_context
 from zerver.lib.outgoing_webhook import do_rest_call, get_outgoing_webhook_service_handler
 from zerver.models import get_bot_services
 from zulip import Client
-from zerver.lib.bot_lib import EmbeddedBotHandler
+from zerver.lib.bot_lib import EmbeddedBotHandler, get_bot_handler
 
 import os
 import sys
@@ -476,12 +476,6 @@ class EmbeddedBotWorker(QueueProcessingWorker):
         # type: (UserProfile) -> EmbeddedBotHandler
         return EmbeddedBotHandler(user_profile)
 
-    def get_bot_handler(self, service):
-        # type: (Service) -> Any
-        bot_module_name = 'zulip_bots.bots.%s.%s' % (service.name, service.name)
-        bot_module = importlib.import_module(bot_module_name)  # type: Any
-        return bot_module.handler_class()
-
     # TODO: Handle stateful bots properly
     def get_state_handler(self):
         # type: () -> StateHandler
@@ -497,7 +491,7 @@ class EmbeddedBotWorker(QueueProcessingWorker):
         # TODO: Do we actually want to allow multiple Services per bot user?
         services = get_bot_services(user_profile_id)
         for service in services:
-            self.get_bot_handler(service).handle_message(
+            get_bot_handler(str(service.name)).handle_message(
                 message=message,
                 bot_handler=self.get_bot_api_client(user_profile),
                 state_handler=self.get_state_handler())