swh.lister.core: Add tests for simple lister

There were previously no tests for the listers which are using the class SimpleLister(like pypi) Refactored test_lister.py of lister core to accommodate tests for SimpleLister keeping the tests undisturbed for other listers.

Also, Add tests for pypi lister Closes #1890 (closed)

---

Migrated from D1733 (view on Phabricator)