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

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

callback/src/test/java/com/iluwatar/callback/CallbackTest.java

@@ -34,7 +34,7 @@ import org.junit.jupiter.api.Test;
  * <p>
  * Could be done with mock objects as well where the call method call is verified.
  */
-public class CallbackTest {
+class CallbackTest {
 
   private Integer callingCount = 0;