[ticket/7580] Add test for IF {VAR} is defined in templates

http://tracker.phpbb.com/browse/PHPBB3-7580
This test demonstrates that is possible to test if a variable exists
and then use it if it does, similar to using isset(), but in 
template/twig syntax.
`<!— IF VAR is defined —>{VAR}<!— ENDIF —>`

PHPBB3-7580