[ticket/13008] Use an absolute path with the FileLocator for the routing files

PHPBB3-13008
Merge pull request #2905 from Nicofuma/ticket/13008

[ticket/13008] Use an absolute path with the FileLocator for the routing files

* Nicofuma/ticket/13008:
  [ticket/13008] Add functionnal test
  [ticket/13008] Use an absolute path with the FileLocator for the routing files