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

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

game-loop/src/test/java/com/iluwatar/gameloop/AppTest.java

@@ -31,7 +31,7 @@ import org.junit.jupiter.api.Test;
 /**
  * App unit test class.
  */
-public class AppTest {
+class AppTest {
 
   @Test
   void shouldExecuteApplicationWithoutException() {

game-loop/src/test/java/com/iluwatar/gameloop/FixedStepGameLoopTest.java

@@ -33,17 +33,17 @@ import org.junit.jupiter.api.BeforeEach;
 /**
  * FixedStepGameLoop unit test class.
  */
-public class FixedStepGameLoopTest {
+class FixedStepGameLoopTest {
 
   private FixedStepGameLoop gameLoop;
 
   @BeforeEach
-  public void setup() {
+  void setup() {
     gameLoop = new FixedStepGameLoop();
   }
 
   @AfterEach
-  public void tearDown() {
+  void tearDown() {
     gameLoop = null;
   }
 

game-loop/src/test/java/com/iluwatar/gameloop/GameControllerTest.java

@@ -35,12 +35,12 @@ class GameControllerTest {
   private GameController controller;
 
   @BeforeEach
-  public void setup() {
+  void setup() {
     controller = new GameController();
   }
 
   @AfterEach
-  public void tearDown() {
+  void tearDown() {
     controller = null;
   }