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

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

promise/src/test/java/com/iluwatar/promise/PromiseTest.java

@@ -50,7 +50,7 @@ class PromiseTest {
   private Promise<Integer> promise;
 
   @BeforeEach
-  public void setUp() {
+  void setUp() {
     executor = Executors.newSingleThreadExecutor();
     promise = new Promise<>();
   }