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

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

strangler/src/test/java/com/iluwatar/strangler/HalfSourceTest.java

@@ -32,7 +32,7 @@ import org.junit.jupiter.api.Test;
 /**
  * Test methods in HalfSource
  */
-public class HalfSourceTest {
+class HalfSourceTest {
   private static final HalfSource source = new HalfSource();
 
   @Test

strangler/src/test/java/com/iluwatar/strangler/NewSourceTest.java

@@ -32,7 +32,7 @@ import org.junit.jupiter.api.Test;
 /**
  * Test methods in NewSource
  */
-public class NewSourceTest {
+class NewSourceTest {
   private static final NewSource source = new NewSource();
 
   @Test

strangler/src/test/java/com/iluwatar/strangler/OldSourceTest.java

@@ -31,7 +31,7 @@ import org.junit.jupiter.api.Test;
 /**
  * Test methods in OldSource
  */
-public class OldSourceTest {
+class OldSourceTest {
   private static final OldSource source = new OldSource();
 
   @Test