Add test `Batch.get_rewards_source`

[Alex]

          Since this is a new method in the Batch class, I think it would be a good idea to add the corresponding tests in the batch tests: https://github.com/alexhernandezgarcia/gflownet/blob/new_fl_loss/tests/gflownet/utils/test_batch.py

Originally posted by @alexhernandezgarcia in #253 (comment)