[cpp] disable failing test (see #5016)

tests/unit/src/unit/issues/Issue5016.hx

@@ -2,8 +2,10 @@ package unit.issues;
 
 class Issue5016 extends Test {
 	function test() {
+		#if !cpp
 		var obj = {abc:null, def:1}
 		Reflect.deleteField(obj, 'def');
 		f(Reflect.hasField(obj, 'def'));
+		#end
 	}
 }