CRM-17860 - Civi\Test - Add mixins for Headless, Hook, Transaction, and EndToEnd functionality.

To describe the environment in which you want to run a test, add an
interface to the test class, e.g.

 * `HeadlessInterface` (along with optional `HookInterface` and `TransactionalInterface`)
 * `EndToEndInterface`

This is an alternative to other approaches like:

 * Extending a gigantic base-class (CiviUnitTestCase, CiviSeleniumTestCase)
 * Copy/paste a bunch of boiler-plate into your own class.