test: fix main file long number issue

subjects/java/checkpoints/digitlen/ExerciseRunner.java

@@ -1,5 +1,5 @@
 public class ExerciseRunner {
     public static void main(String[] args) {
-        System.out.println(Digitlen.digitlen(9989898878));
+        System.out.println(Digitlen.digitlen(9989898878L));
     }
 }