vault.tests: Make postgresql fixture faster
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 (min) | after (s) | Using |
|---------+--------------+-----------+-----------------------|
| laptop | 1:11.02 | 20.811 | `time tox` |
| jenkins | 1:14 | 26 | jenkins output report |
|---------+--------------+-----------+-----------------------|
Test Plan
tox
Migrated from D4310 (view on Phabricator)