Fix policy tests

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

 def basic_mock_brain():
     mock_brain = MagicMock()
     mock_brain.vector_action_space_type = "continuous"
+    mock_brain.vector_observation_space_size = 1
+    mock_brain.vector_action_space_size = [1]
     return mock_brain