Set object id when calling objstorage.add

We are removing support for the objstorage computing the object id itself.

Related to swh-objstorage#2309

---

Migrated from D8008 (view on Phabricator)

swh/dataset/test/test_orc.py

@@ -327,7 +327,7 @@ def test_export_content_with_data(monkeypatch, tmpdir):
     obj_type = "content"
     objstorage = get_objstorage("memory")
     for content in TEST_OBJECTS[obj_type]:
-        objstorage.add(content.data)
+        objstorage.add(content=content.data, obj_id=content.sha1)
 
     def get_objstorage_mock(**kw):
         if kw.get("cls") == "mock":