remove final from class name

see #571

exercises/flatten-array/src/test/java/FlattenerTest.java

@@ -10,7 +10,7 @@ import static org.junit.Assert.assertEquals;
 /*
  * version: 1.1.0
  */
-public final class FlattenerTest {
+public class FlattenerTest {
 
     private Flattener flattener;