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

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

singleton/src/test/java/com/iluwatar/singleton/EnumIvoryTowerTest.java

@@ -29,7 +29,7 @@ package com.iluwatar.singleton;
  *
  * @author Jeroen Meulemeester
  */
-public class EnumIvoryTowerTest extends SingletonTest<EnumIvoryTower> {
+class EnumIvoryTowerTest extends SingletonTest<EnumIvoryTower> {
 
   /**
    * Create a new singleton test instance using the given 'getInstance' method.

singleton/src/test/java/com/iluwatar/singleton/InitializingOnDemandHolderIdiomTest.java

@@ -29,7 +29,7 @@ package com.iluwatar.singleton;
  *
  * @author Jeroen Meulemeester
  */
-public class InitializingOnDemandHolderIdiomTest
+class InitializingOnDemandHolderIdiomTest
     extends SingletonTest<InitializingOnDemandHolderIdiom> {
 
   /**

singleton/src/test/java/com/iluwatar/singleton/IvoryTowerTest.java

@@ -29,7 +29,7 @@ package com.iluwatar.singleton;
  *
  * @author Jeroen Meulemeester
  */
-public class IvoryTowerTest extends SingletonTest<IvoryTower> {
+class IvoryTowerTest extends SingletonTest<IvoryTower> {
 
   /**
    * Create a new singleton test instance using the given 'getInstance' method.

singleton/src/test/java/com/iluwatar/singleton/ThreadSafeLazyLoadedIvoryTowerTest.java

@@ -29,7 +29,7 @@ package com.iluwatar.singleton;
  *
  * @author Jeroen Meulemeester
  */
-public class ThreadSafeLazyLoadedIvoryTowerTest
+class ThreadSafeLazyLoadedIvoryTowerTest
     extends SingletonTest<ThreadSafeLazyLoadedIvoryTower> {
 
   /**