perfect-numbers: remove "final" from class declaration

see #571

exercises/perfect-numbers/src/test/java/NaturalNumberTest.java

@@ -9,7 +9,7 @@ import static org.junit.Assert.assertEquals;
  * version: 1.0.0
  * https://github.com/exercism/x-common/blob/19d0c7714ce664a1ad762af624c17f8e269fa8b2/exercises/perfect-numbers/canonical-data.json
  */
-public final class NaturalNumberTest {
+public class NaturalNumberTest {
 
     /*
      * See https://github.com/junit-team/junit4/wiki/Rules for information on JUnit Rules in general and