tests: make test_forks teardown more stable It would sometimes fail like: kallithea/tests/functional/test_forks.py:35: in teardown_method Session().delete(self.u1) data/env/lib/python2.7/site-packages/sqlalchemy/orm/session.py:1871: in delete self._delete_impl(state, instance, head=True) data/env/lib/python2.7/site-packages/sqlalchemy/orm/session.py:1888: in _delete_impl self.identity_map.add(state) data/env/lib/python2.7/site-packages/sqlalchemy/orm/identity.py:149: in add orm_util.state_str(state), state.key)) E InvalidRequestError: Can't attach instance <User at 0x7f93d2f81a10>; another instance with key (<class 'kallithea.model.db.User'>, (10,), None) is already present in this session.