[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

tests/template/templates/if.html

@@ -19,3 +19,7 @@ false
 <!-- IF S_TEST !== false -->
 !false
 <!-- ENDIF -->
+
+<!-- IF VALUE_TEST is defined -->
+{VALUE_TEST}
+<!-- ENDIF -->