[ticket/12099] Fix several issues in path_helper test

PHPBB3-12099
[ticket/12099] Clean some paths before using them

PHPBB3-12099
[ticket/12099] Correctly fix go back to root before prepending the root path

This allows moving app.php outside of phpBB and still work as expected

PHPBB3-12099
[ticket/12099] Clean paths in tests

PHPBB3-12099
[ticket/12099] Break clean_path tests with a simple test

PHPBB3-12099
[ticket/12099] Fix clean_path() ".." stripping when previous directory was "."

PHPBB3-12099