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

See issue #571

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

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