docstr: Test that a trajectory suffix has the correct length.
code: def test_trajectory_suffix_len(self):
      """Test that a trajectory suffix has the correct length."""
      tr = rl_task.Trajectory(observation=0)
      for _ in range(5):
          self._extend(tr)
      tr_suffix = tr.suffix(length=3)
      self.assertLen(tr_suffix, 3)
  
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 7 for calling the TaskTest obj's underlying member method code with example usage: obj.test_trajectory_suffix_len() without return types