refactor: remove code smell java:S5786 (#2159)

https://sonarcloud.io/organizations/iluwatar/rules?open=java%3AS5786&rule_key=java%3AS5786

null-object/src/test/java/com/iluwatar/nullobject/TreeTest.java

@@ -44,17 +44,17 @@ import org.slf4j.LoggerFactory;
  *
  * @author Jeroen Meulemeester
  */
-public class TreeTest {
+class TreeTest {
 
   private InMemoryAppender appender;
 
   @BeforeEach
-  public void setUp() {
+  void setUp() {
     appender = new InMemoryAppender();
   }
 
   @AfterEach
-  public void tearDown() {
+  void tearDown() {
     appender.stop();
   }