Tests: assign the world to WorldTestBase, and a default player field (#2385)

* Tests: assign the World to WorldTestBase and add a player field (because I like typing self.player far more than random 1's all over the place)

* more accurate docstring for world and multiworld

* use self.player within the class

worlds/messenger/test/__init__.py

@@ -1,6 +1,7 @@
 from test.TestBase import WorldTestBase
+from .. import MessengerWorld
 
 
 class MessengerTestBase(WorldTestBase):
     game = "The Messenger"
-    player: int = 1
+    world: MessengerWorld