[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
[ticket/12099] Deduplicate path generation

PHPBB3-12099
[ticket/12099] Prepend ./ to path to fix assets

PHPBB3-12099
[ticket/12099] Fix correction in path_helper test

PHPBB3-12099
Merge remote-tracking branch 'nickvergessen/ticket/12099' into develop-ascraeus

* nickvergessen/ticket/12099:
  [ticket/12099] Fix correction in path_helper test
  [ticket/12099] Prepend ./ to path to fix assets
  [ticket/12099] Deduplicate path generation
  [ticket/12099] Fix clean_path() ".." stripping when previous directory was "."
  [ticket/12099] Break clean_path tests with a simple test
  [ticket/12099] Clean paths in tests
  [ticket/12099] Correctly fix go back to root before prepending the root path
  [ticket/12099] Clean some paths before using them
  [ticket/12099] Fix several issues in path_helper test