fixed default trainer_util test to expect brain_name

ml-agents/mlagents/trainers/tests/test_trainer_util.py

         load_model=False,
         seed=42,
     )
-    trainer_factory.generate(brain_parameters)
+    trainer_factory.generate(brain_parameters.brain_name)
 
 
 def test_raise_if_no_config_for_brain():