Re-enables TestSSH/RemoteForward_Unix_Signal and addresses the underlying race: we were not closing the remote forward on context expiry, only the session and connection.

However, there is still a more fundamental issue in that we don't have the ability to ensure that TCP sessions are properly terminated before tearing down the Tailnet conn. This is due to the assumption in the sockets API, that the underlying IP interface is long
lived compared with the TCP socket, and thus closing a socket returns immediately and does not wait for the TCP termination handshake --- that is handled async in the tcpip stack. However, this assumption does not hold for us and tailnet, since on shutdown,
we also tear down the tailnet connection, and this can race with the TCP termination.

Closing the remote forward explicitly should prevent forward state from accumulating, since the Close() function waits for a reply from the remote SSH server.

I've also attempted to workaround the TCP/tailnet issue for --stdio by using CloseWrite() instead of Close(). By closing the write side of the connection, half-close the TCP connection, and the server detects this and closes the other direction, which then
triggers our read loop to exit only after the server has had a chance to process the close.

TODO in a stacked PR is to implement this logic for vscodessh as well.