feat: add "while-not-null" test

tests/programs/pos_while_not_null.fzs

@@ -0,0 +1,28 @@
+class Some {
+  Some next;
+}
+
+Some l4 = new Some;
+
+Some l3 = new Some;
+l3.next = l4;
+
+Some l2 = new Some;
+l2.next = l3;
+
+Some l1 = new Some;
+l1.next = l2;
+
+Some root = new Some;
+root.next = l1;
+
+Some obj = root;
+while (obj != null)
+{
+  obj = obj.next;
+  printline("Now object is:");
+  printline(obj);
+}
+
+printline(obj);
+