API: Notification Helpers tests

[deandrebaker]

We need to add tests for the following:

• create_tagged_users_notification creates and passes the correct notification object for a tagged notification to the send_push_notifications function.
• create_connection_request_notification creates and passes the correct notification object for a connection notification to the send_push_notifications function.
• create_message_notification creates and passes the correct notification object for a message notification to the send_push_notifications function.
• create_liked_shared_item_notification creates and passes the correct notification object for a like notification to the send_push_notifications function.
• send_push_notifications populates the push_messages array with the correct push notification messages and passes it to the PushClient().publish_multiple method.