What to use for firewall testing (port opened or not)

You could use a higher-level software that generates iptables rules, like shorewall. It has a command 'shorewall check' that checks the consistency and errors in your rules.

telnet is not netcat. The telnet protocol is more than raw TCP. Among other things it can have a number of options, and the "noise" you're seeing is the negotiation of these options between your client and the server. When you specify a port you don't see any noise because according to the manual:

When connecting to a non-standard port, telnet omits any automatic initiation of TELNET options. When the port number is preceded by a minus sign, the initial option negotiation is done.

So apparently your implementation of telnet disables option negotiation when you specify a port (even when the port is 23), and re-enables it when the port is preceded by a minus sign.

On a more general note, it's generally safe to forget about telnet these days. Use netcat instead if you need a simple plain TCP client (or server, for that matter).