[ticket/11620] Rename provider -> mock_auth_provider

Rename the class and file name to better match
what the class is mocking, as well as implement the
interface of that class.

PHPBB3-11620

tests/mock/auth_provider.php

@@ -14,7 +14,8 @@
  * See interface here:
  *    includes/auth/provider/interface.php
  */
-class phpbb_provider {
+class phpbb_mock_auth_provider implements phpbb_auth_provider_interface
+{
 
 	function init()
 	{