update tests with new default server ip ('')

and verify that starting a Spawner sets ip to localhost

jupyterhub/tests/conftest.py

@@ -22,7 +22,7 @@ def db():
     """Get a db session"""
     global _db
     if _db is None:
-        _db = orm.new_session_factory('sqlite:///:memory:', echo=True)()
+        _db = orm.new_session_factory('sqlite:///:memory:')()
         user = orm.User(
             name=getuser(),
             server=orm.Server(),