largest-series-product: remove "final" from class declaration

See #571

exercises/largest-series-product/src/test/java/LargestSeriesProductCalculatorTest.java

@@ -5,7 +5,7 @@ import org.junit.rules.ExpectedException;
 
 import static org.junit.Assert.assertEquals;
 
-public final class LargestSeriesProductCalculatorTest {
+public class LargestSeriesProductCalculatorTest {
 
     /*
      * See https://github.com/junit-team/junit4/wiki/Rules for information on JUnit Rules in general and