corrected env observation test

tests/test_env_observation_builder.py

@@ -82,7 +82,7 @@ def test_global_obs():
 
     # If this assertion is wrong, it means that the observation returned
     # places the agent on an empty cell
-    assert(np.sum(rail_map * global_obs[0][1][:, :, 0]) > 0)
+    assert(np.sum(rail_map * global_obs[0][1][:, :, :4].sum(2)) > 0)
 
 
 def main():