[ticket/12413] Fatal Error log created for feed.php file 

https://tracker.phpbb.com/browse/PHPBB3-12413
http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45475

I also added functional tests for the feeds and fix some bugs encountered while I was writing the tests.

Notice: Some tests are disabled (commented) until PHPBB3-12418 (#2331) and PHPBB3-12421 (#2330) are fixed and merged

PHPBB3-12413