Add stacked_trajectory_from_transition to trajectories/test_utils.py.
Add stacked_trajectory_from_transition to trajectories/test_utils.py.

This helps us build trainable Trajectory objects in tests.

PiperOrigin-RevId: 248371353
Change-Id: Ib2cc06fc046dae50819527452c1e1eb7f2a35352

tensorflow/agents,tensorflow/agents