SQLStore: Fix wrong usage of xorm's insert functions in tests (#63850)

* SQLStore: Fix InsertId

* Prefs: Fix Insert return value

* Fix tests

* Add guidelines

pkg/services/preference/prefimpl/xorm_store.go

@@ -73,7 +73,8 @@ func (s *sqlStore) Insert(ctx context.Context, cmd *pref.Preference) (int64, err
 	var ID int64
 	var err error
 	err = s.db.WithTransactionalDbSession(ctx, func(sess *db.Session) error {
-		ID, err = sess.Insert(cmd)
+		_, err = sess.Insert(cmd)
+		ID = cmd.ID
 		return err
 	})
 	return ID, err