Make system auth tests work with EAS.

Summary:
They weren't working with EAS, because only inbox.auth.handler_from_provider
turns an 'unknown' provider into 'eas'.

Test Plan:
Run py.test tests/system/test_auth, as well as bin/inbox-auth, on
test accounts for Gmail, Outlook, Yahoo and EAS. Also verify that none of the
affected code is called from redwood.

Reviewers: charles

Reviewed By: charles

Differential Revision: https://review.inboxapp.com/D355