Update Mail test adapter for the newer adapter API and make all tests pass

Summary: Depends on D19956. Ref T920. Move the TestAdapter to the new API and adjust a couple of tests for the changes.

Test Plan: All tests now pass.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D19957