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

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

poison-pill/src/test/java/com/iluwatar/poison/pill/ConsumerTest.java

@@ -42,17 +42,17 @@ import org.slf4j.LoggerFactory;
  *
  * @author Jeroen Meulemeester
  */
-public class ConsumerTest {
+class ConsumerTest {
 
   private InMemoryAppender appender;
 
   @BeforeEach
-  public void setUp() {
+  void setUp() {
     appender = new InMemoryAppender(Consumer.class);
   }
 
   @AfterEach
-  public void tearDown() {
+  void tearDown() {
     appender.stop();
   }
 

poison-pill/src/test/java/com/iluwatar/poison/pill/PoisonMessageTest.java

@@ -35,7 +35,7 @@ import org.junit.jupiter.api.Test;
  *
  * @author Jeroen Meulemeester
  */
-public class PoisonMessageTest {
+class PoisonMessageTest {
 
   @Test
   void testAddHeader() {

poison-pill/src/test/java/com/iluwatar/poison/pill/ProducerTest.java

@@ -41,7 +41,7 @@ import org.mockito.ArgumentCaptor;
  *
  * @author Jeroen Meulemeester
  */
-public class ProducerTest {
+class ProducerTest {
 
   @Test
   void testSend() throws Exception {

poison-pill/src/test/java/com/iluwatar/poison/pill/SimpleMessageTest.java

@@ -37,7 +37,7 @@ import org.junit.jupiter.api.Test;
  *
  * @author Jeroen Meulemeester
  */
-public class SimpleMessageTest {
+class SimpleMessageTest {
 
   @Test
   void testGetHeaders() {