bracket-push: remove "public final" from test class

See issue #571

exercises/bracket-push/src/test/java/BracketCheckerTest.java

@@ -5,7 +5,7 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
-public final class BracketCheckerTest {
+class BracketCheckerTest {
 
     @Test
     public void testPairedSquareBrackets() {