This reuse the swh.core.db.db_utils postgresql_fact fixture (which inits the db
then truncate tables in between test calls). This makes the tests faster.
Basic benchmark:
```
|---------+----------+--------|
| machine | before | after |
|---------+----------+--------|
| laptop | 1:11.02 | 20.811 |
| jenkins | 1min 14s | 26s |
|---------+----------+--------|
```