minesweeper: remove "final" from class declaration

see #571

exercises/minesweeper/src/test/java/MinesweeperBoardTest.java

@@ -9,7 +9,7 @@ import java.util.List;
 
 import static org.junit.Assert.assertEquals;
 
-public final class MinesweeperBoardTest {
+public class MinesweeperBoardTest {
 
     /*
      * See https://github.com/junit-team/junit4/wiki/Rules for information on JUnit Rules in general and