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

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

thread-pool/src/test/java/com/iluwatar/threadpool/CoffeeMakingTaskTest.java

@@ -29,7 +29,7 @@ package com.iluwatar.threadpool;
  *
  * @author Jeroen Meulemeester
  */
-public class CoffeeMakingTaskTest extends TaskTest<CoffeeMakingTask> {
+class CoffeeMakingTaskTest extends TaskTest<CoffeeMakingTask> {
 
   /**
    * Create a new test instance

thread-pool/src/test/java/com/iluwatar/threadpool/PotatoPeelingTaskTest.java

@@ -29,7 +29,7 @@ package com.iluwatar.threadpool;
  *
  * @author Jeroen Meulemeester
  */
-public class PotatoPeelingTaskTest extends TaskTest<PotatoPeelingTask> {
+class PotatoPeelingTaskTest extends TaskTest<PotatoPeelingTask> {
 
   /**
    * Create a new test instance

thread-pool/src/test/java/com/iluwatar/threadpool/WorkerTest.java

@@ -36,7 +36,7 @@ import org.junit.jupiter.api.Test;
  *
  * @author Jeroen Meulemeester
  */
-public class WorkerTest {
+class WorkerTest {
 
   /**
    * Verify if a worker does the actual job