change: remove "final" from test class declaration

Closes: #571

exercises/change/src/test/java/ChangeCalculatorTest.java

 import static java.util.Collections.singletonList;
 import static org.junit.Assert.assertEquals;
 
-public final class ChangeCalculatorTest {
+public class ChangeCalculatorTest {
 
     /*
      * See https://github.com/junit-team/junit4/wiki/Rules for information on JUnit Rules in general and