Regression test for T441

Summary:
This is a very basic regression test for T441. Two things to note:
- To make things easier to test, I moved the thread fetching code into a separate function, `fetch_similar_threads`
- I had to resort to monkeypatching a function FolderSyncMonitor needed to get this to work. I think it's okay since we only instantiate a monitor to call `fetch_similar_threads`.

Test Plan: Ran the test.

Reviewers: charles, emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D472