[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/template_test.php

@@ -90,6 +90,13 @@ class phpbb_template_template_test extends phpbb_template_template_test_case
 				array(),
 				'03!false',
 			),
+			array(
+				'if.html',
+				array('VALUE_TEST' => 'value'),
+				array(),
+				array(),
+				'03!falsevalue',
+			),
 			array(
 				'loop.html',
 				array(),

tests/template/templates/if.html

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