ocr-numbers: remove "final" keyword from test declaration

See #571

exercises/ocr-numbers/src/test/java/OpticalCharacterReaderTest.java

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