[ticket/11761] Serve blank file locally in functional test

Example.org no longer serves blank responses, failing functional tests.

this patch creates a  blank file and serve it locally during the test,
instead of hitting the http://example.org servers kindly provided by IANA.

PHPBB3-11761
Merge remote-tracking branch 'asperous/ticket/11761' into develop

* asperous/ticket/11761:
  [ticket/11761] Serve blank file locally in functional test