docstr: Test that trajectory length is equal to the number of observations.
code: def test_trajectory_len(self):
      """Test that trajectory length is equal to the number of observations."""
      tr = rl_task.Trajectory(observation=0)
      for _ in range(5):
          self._extend(tr)
      self.assertLen(tr, 6)
  
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 3 for calling the TaskTest obj's underlying member method code with example usage: obj.test_trajectory_len() without return types