Earlier, when we used 'self.send_message()' in the backend tests,
the sent message was not marked as read for the sender.
Reason: To set the read flag, we have to check if
'message.sent_by_human()'. It returns False because the
'sending_client' for tests is "test suite" and the 'sent_by_human'
function doesn't enlist the "test suite" client name as a human client.
This commit adds "test suite" to that list.
Also fixes a bug in when apply_unread_message_event was called that
was revealed by this change.
c349d1137c