Add integration tests for Zulip

[Kobzol]

#784 added a Zulip client, but I haven't yet implemented tests for it. It would be nice to implement a mocked Zulip server, same as we do for GitHub, and use it to test the sent Zulip messages.

[cuba0001]

Can I take it?

[Kobzol]

Sure, feel free to take a look :)