Add .is_closed() for FakeSnowflakeConnection

[ilietrasca]

If someone want to use write some code where is reusing connection the FakeSnowflakeConnection is causing problems because dose not have the is_closed check which is an easy to add feature.

[ilietrasca]

I tried to follow that instructions for local dev, but the format is not working and for a small change I'm formatting the entire file, anyway.

The if someone can change FakeSnowflakeConnection:

    def is_closed(self) -> bool:
        return self._duck_conn is None

And test, the connection can will always be true, but like this will help replicating SnowflakeConnection :

def test_is_closed_present(conn: snowflake.connector.SnowflakeConnection):
    assert conn.is_closed() is False