[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
Merge remote-tracking branch 'vsephpbb/ticket/7580' into develop

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