This reuse the swh.core.db.db_utils.SwhDatabasejanitor which allows to truncate
tables in between test calls. This makes the tests faster.
on my machine, with this:
tox 13.50s user 2.88s system 78% cpu 20.811 total
on my machine, without:
tox 22.35s user 5.83s system 39% cpu 1:11.02 total
We'll see more clearly once D4300 lands (by checking jenkins then)