feat: started to test GameManager class

src/test/java/hepia/GameManagerTest.java

 package hepia;
 
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+
 public class GameManagerTest {
+
+    @Test
+    public void invalidNumberOfPlayers() {
+        int nbHumanPlayers = 6;
+        int nbBasicAi = 1;
+        int nbAdvancedAi = 1;
+        int nbDecks = 3;
+
+        assertThrows(RuntimeException.class, () -> new GameManager(Game.BLACKJACK, nbHumanPlayers, nbBasicAi, nbAdvancedAi, nbDecks));
+    }
 }