[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
[ticket/11550] Fixtures should only be directories not files

PHPBB3-11550
[ticket/11550] We use a different fixture set for extension_acp_test.php

PHPBB3-11550
[ticket/11550] Move comments to correct function

PHPBB3-11550
[ticket/11550] Specify a valid path so it's clearer that it must be a path

PHPBB3-11550
Merge remote-tracking branch 'nickvergessen/ticket/11550' into develop

* nickvergessen/ticket/11550:
  [ticket/11550] Specify a valid path so it's clearer that it must be a path
  [ticket/11550] Move comments to correct function
  [ticket/11550] We use a different fixture set for extension_acp_test.php
  [ticket/11550] Fixtures should only be directories not files
  [ticket/11550] Use new functionality from the test case helpers
  [ticket/11550] Move functionality for copying/restoring to test case helpers
  [ticket/11550] Fix copying the fixtures in extension_permission_lang_test.php