Mark test_identify_closure as timing-sensitive

Mark test_identify_closure as timing-sensitive

```
____________________________ test_identify_closure _____________________________

conn_cls = <class 'psycopg.Connection'>, dsn = 'host=127.0.0.1 user=psycopg'

    @pytest.mark.crdb_skip("pg_terminate_backend")
    @pytest.mark.slow
    def test_identify_closure(conn_cls, dsn):
        def closer():
            time.sleep(0.2)
            conn2.execute("select pg_terminate_backend(%s)", [conn.pgconn.backend_pid])

        conn = conn_cls.connect(dsn)
        conn2 = conn_cls.connect(dsn)
        try:
            t = threading.Thread(target=closer)
            t.start()
            t0 = time.time()
            try:
                with pytest.raises(psycopg.OperationalError):
                    conn.execute("select pg_sleep(1.0)")
                t1 = time.time()
>               assert 0.2 < t1 - t0 < 0.4
E               assert 0.2 < (1697715164.8614173 - 1697715164.6617775)

tests/test_concurrency.py:231: AssertionError
```