[ticket/11550] Fix copying the fixtures in extension_permission_lang_test.php

PHPBB3-11550
[ticket/11550] Move functionality for copying/restoring to test case helpers

PHPBB3-11550
[ticket/11550] Use new functionality from the test case helpers

Instead of duplicating the extension copy code, it is now in the helpers.
So we remove the code from existing tests.

PHPBB3-11550