Better (but not faster) system tests

Summary:
Charles,

I made a couple changes to the system tests:
- for_all_available_providers now sets up and teardowns test accounts (fixes T311).
- I extended test_sending to archive the email and then trash it.

I found a race condition in mailsync - sending would crash if tried to early in the sync because the folder mapping isn't stored in the db. I added a 30 seconds sleep to handle this. I don't know if it's worth it to go after this bug.

Test Plan: Ran the tests for a couple accounts.

Reviewers: charles

Reviewed By: charles

Maniphest Tasks: T311, T315

Differential Revision: https://review.inboxapp.com/D360