There are missing bindings for qrels - for example, I can't get TREC 20 DL qrels. They're in Anserini, just not properly exposed in Pyserini.

We should add test cases:
https://github.com/castorini/pyserini/blob/master/tests/test_load_topics.py

Rename test_load_topics.py to test_load_topics_and_qrels.py and make sure each topics/qrels pair works...