log.Origin: return self on __enter__

Allows using 'with some_origin() as foo:' constructs.
Not used actively, but is sometimes useful during debugging sessions.

Change-Id: I7a6463ee39761775305dd2272c24f248552db4ad

src/osmo_gsm_tester/log.py

@@ -332,8 +332,9 @@ class Origin:
 
     def __enter__(self):
         if not self.set_child_of(Origin._global_current_origin):
-            return
+            return self
         Origin._global_current_origin = self
+        return self
 
     def __exit__(self, *exc_info):
         rc = None