docstr: Test that to_np adds a specified margin.
code: def test_trajectory_to_np_adds_margin(self):
      """Test that to_np adds a specified margin."""
      tr = rl_task.Trajectory(observation=2)
      for _ in range(2):
          self._extend(tr, new_observation=2)
      tr_np = tr.to_np(margin=2)
      np.testing.assert_array_equal(tr_np.observation, [2, 2, 2, 0])
      np.testing.assert_array_equal(tr_np.mask, [1, 1, 0, 0])
  
imports: import os
  from absl.testing import absltest
  import gym
  import numpy
  from trax import test_utils
  from trax.rl import task
label_desc: Use image node_id 12 for calling the TaskTest obj's underlying member method code with example usage: obj.test_trajectory_to_np_adds_margin() without return types