Fix TestTrainAgentAsync

[toslunar]

• https://travis-ci.org/chainer/chainerrl/jobs/463226829

          # A hook receives (env, agent, step)
          for i, call in enumerate(hook.call_args_list):
              args, kwargs = call
              self.assertTrue(any(args[0] == env for env in envs))
              self.assertEqual(args[1], agent)
              # step starts with 1
  >           self.assertEqual(args[2], i + 1)
  E           AssertionError: 5 != 4
  

• https://travis-ci.org/chainer/chainerrl/jobs/463226831

          if self.num_envs == 1:
              self.assertEqual(hook.call_count, steps)
          elif self.num_envs > 1:
  >           self.assertGreater(hook.call_count, steps)
  E           AssertionError: 3 not greater than 5
  

[muupan]

Good catch!