Make the "Test" adapter support both SMS and email

Summary:
Depends on D20012. Ref T920. If you have a test adapter configured, it should swallow messages and prevent them from ever hitting a lower-priority adapter.

Make the test adapter support SMS so this actually happens.

Test Plan: Ran bin/mail send-test --type sms ... with a test adapter (first) and a Twilio adapter (second). Got SMS swallowed by test adapter instead of live SMS messages.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D20013