loader.tar: Add some tests and remove dead code

loader.tar.utils: Add tests to random_block function swh.loader.tar.utils: Remove duplicated grouper definition

Related swh-environment#1411 (closed) Depends on swh-core!25 (closed)

Test Plan

tox

The tests won't build up until the diff is landed and a new tag is pushed though.

---

Migrated from D794 (view on Phabricator)