Basic test case for labels

Summary:
Emfree,

I've got a basic system test for labels. The idea is to use a crispin client to modify the IMAP backend and check that mailsync picks up the changes. So far, I haven't been able to find a bug but I'm sure I'll find something ;)

Test Plan: Yo dawg I herd you liked tests so I put a test in your test

Reviewers: emfree

Reviewed By: emfree

Projects: #sync-engine

Differential Revision: https://review.inboxapp.com/D541