rectangles: remove "final" from class declaration

see #571

exercises/rectangles/src/test/java/RectangleCounterTest.java

@@ -4,7 +4,7 @@ import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
 
-public final class RectangleCounterTest {
+public class RectangleCounterTest {
 
     private RectangleCounter rectangleCounter;