add findGameById-Test

fh.fd.ci.server/src/test/java/de/fd/fh/server/game/GameServiceTest.java

@@ -77,4 +77,16 @@ class GameServiceTest
         then(repository).should().save(any(Game.class));
         then(repository).shouldHaveNoMoreInteractions();
     }
+
+    @Test
+    void given_storedGame_when_findGameById_should_returnGame()
+    {
+        final GameRepository repository = when(mock(GameRepository.class).findByGameId(any(GameId.class)))
+                .thenReturn(new Game(null, null, null, null, null, null))
+                .getMock();
+
+        final Game result = new GameService(repository).findGameById(GameId.of("12345"));
+
+        assertNotNull(result);
+    }
 }