queen-attack: remove "final" from class declaration

See #571

exercises/queen-attack/src/test/java/QueenAttackCalculatorTest.java

@@ -6,7 +6,7 @@ import org.junit.rules.ExpectedException;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
-public final class QueenAttackCalculatorTest {
+public class QueenAttackCalculatorTest {
 
     /*
      * See https://github.com/junit-team/junit4/wiki/Rules for information on JUnit Rules in general and