Add old tests from imitation envs to seals

[Rocamonde]

Old tests:

def test_model_envs(env):
    """Smoke test for each of the ModelBasedEnv methods with type checks.
    Args:
        env: The environment to test.
    Raises:
        AssertionError if test fails.
    """
    state = env.initial_state()
    assert env.pomdp_state_space.contains(state)

    action = env.action_space.sample()
    new_state = env.transition(state, action)
    assert env.pomdp_state_space.contains(new_state)

    reward = env.reward(state, action, new_state)
    assert isinstance(reward, float)

    done = env.terminal(state, 0)
    assert isinstance(done, bool)

    obs = env.obs_from_state(state)
    assert env.pomdp_observation_space.contains(obs)
    next_obs = env.obs_from_state(new_state)
    assert env.pomdp_observation_space.contains(next_obs)