Add unit tests for mail failover behaviors when multiple mailers are configured

Summary: Depends on D19006. Ref T13053. Ref T12677. When multiple mailers are configured but one or more fail, test that we recover (or don't) appropriately.

Test Plan: Ran unit tests.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053, T12677

Differential Revision: https://secure.phabricator.com/D19007