[ticket/10844] Add phpbb_root_path to phpbb_style_extension_path_provider

The phpbb_root_path needs to be removed from the style path, before giving
the path to the finder, because the finder prepends it later again and is
therefor unable to find style files when the root path is not ./

PHPBB3-10844
[ticket/10844] Make sure ext_path has no trailing slash

PHPBB3-10844
[ticket/10844] Add unit tests for different root_paths

PHPBB3-10844