Add some test coverage for mailers configuration

Summary: Depends on D19005. Ref T12677. Ref T13053. Tests that turning cluster.mailers into an actual list of mailers more or less works as expected.

Test Plan: Ran unit tests.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053, T12677

Differential Revision: https://secure.phabricator.com/D19006