diamond: remove "final" from test class declaration

see #571

exercises/diamond/src/test/java/DiamondPrinterTest.java

 import static org.hamcrest.CoreMatchers.is;
 import static org.junit.Assert.assertThat;
 
-public final class DiamondPrinterTest {
+public class DiamondPrinterTest {
     private DiamondPrinter diamondPrinter;
     
     @Before