[tests] Verify that Google calendars events are actually synced back

Summary:
This is probably not the prettiest test ever: there's a lot of copy paste from test_events (I'm welcoming your suggestions on how to solve this btw -- I tried to use fixtures and stuff but this just wouldn't work with py.test).

The test already paid itself by catching not one but two bugs!

Test Plan: Ran the test

Reviewers: emfree, kav-ya

Reviewed By: kav-ya

Projects: #testing

Differential Revision: https://review.inboxapp.com/D788