tests: Make check-snapshot utility test function recursively check targetted object exists

Make the swh.loader.tests.check_snapshot function check object references recursively. Making a recursive check of all objects pointed at by the snapshot would alleviate these concerns completely (and for all loaders).

That would allow ensuring that we no longer generate buggy edges (at least from test standpoint first). And if we do, making it catch quickly by our ci.

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Migrated from T2483 (view on Phabricator)