Add test case interface

This interface will then also be implemented by the bundle base test class, at
least reducing the duplication in PHPDoc.
Is this a good name, or should we have `getTestClient()` instead? This becomes especially relevant in the bundle, where `ProxyTestCase` extends Symfony’s `WebTestCase`, which also provides a client (though not through a `getClient()` method). Will these two clients be confusing for users?