docstr: Test that we can get and set the `done` flag of a trajectory.
code: def test_trajectory_done_get_and_set(self):
      """Test that we can get and set the `done` flag of a trajectory."""
      tr = rl_task.Trajectory(observation=123)
      self._extend(tr)
      self.assertFalse(tr.done)
      tr.done = True
      self.assertTrue(tr.done)
  
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 6 for calling the TaskTest obj's underlying member method code with example usage: obj.test_trajectory_done_get_and_set() without return types