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/lib/bot_lib.py

@@ -7,8 +7,10 @@ import signal
 import sys
 import time
 import re
+import importlib
 from zerver.lib.actions import internal_send_message
 from zerver.models import UserProfile
+from zerver.lib.integrations import EMBEDDED_BOTS
 
 from six.moves import configparser
 
@@ -21,6 +23,16 @@ our_dir = os.path.dirname(os.path.abspath(__file__))
 
 from zulip_bots.lib import RateLimit
 
+def get_bot_handler(service_name):
+    # type: (str) -> Any
+
+    # Assert that this service is present in EMBEDDED_BOTS.
+    assert any(service_name == embedded_bot_service.name for embedded_bot_service in EMBEDDED_BOTS)
+
+    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()
+
 class EmbeddedBotHandler(object):
     def __init__(self, user_profile):
         # type: (UserProfile) -> None