secret-handshake: remove "final" keyword from test declaration

See #571

exercises/secret-handshake/src/test/java/HandshakeCalculatorTest.java

 import static java.util.Collections.singletonList;
 import static org.junit.Assert.assertEquals;
 
-public final class HandshakeCalculatorTest {
+public class HandshakeCalculatorTest {
     private HandshakeCalculator handshakeCalculator;