Update length property test for Intl.DateTimeFormat.prototype.format

Per https://github.com/tc39/ecma402/issues/76

test/intl402/DateTimeFormat/prototype/format/12.3.2_1_a_L15.js

@@ -12,4 +12,4 @@ author: Norbert Lindenberg
 includes: [testBuiltInObject.js]
 ---*/
 
-testBuiltInObject(new Intl.DateTimeFormat().format, true, false, [], 0);
+testBuiltInObject(new Intl.DateTimeFormat().format, true, false, [], 1);