test scenes for unit tests should to be generated on the fly rather than checked into VCS

Story

As a developer, I want RailEnvs for unit tests to be generated on the fly rather than check into VCS so that we always get valid RailEnvs (version compatibility).

Acceptance Criteria

• Test files Level_distance_map_shortest_path.pkl and test_002.pkl which are used in unit tests - test_flatland_envs_rail_env_shortest_paths should be generated on the fly and not committed to version control system.
• Enable unit tests tests test_flatland_envs_rail_env_shortest_paths again