Make sure loading a svn repo properly sets the snapshot id in the visit
Since the refactoring of swh.loader.core 0.0.74, so called BufferedLoaders don't set the snapshot in the visit automatically. This loader would load data properly, but fail to record the loaded snapshot into the visit.
Test Plan
running the new tests without the loader change would fail
Migrated from D2698 (view on Phabricator)