added a method to find all ghosts currently seated in the train and added repo structure for tests as well as an empty test class

src/test/java/ch/unibas/dmi/dbis/cs108/gamelogic/klassenstruktur/GameStateTests.java

@@ -0,0 +1,5 @@
+package ch.unibas.dmi.dbis.cs108.gamelogic.klassenstruktur;
+
+public class GameStateTests {
+
+}