Testbed: Use log instead of raw println

[Vrixyz]

Hello,

I am using testbed to live test a 2d game server, It's a handy tool !

That said, my server has an interactive shell, I'm using the log facade logic (with info!(), debug!()...) to display only information related to input and output for the user, and I have a "logmode" reaching out to files where logs are organized.

I'd like to organize testbed logs in a dedicated folder, and that would be easy if testbed used the log facade.

For information, the log implementation I use is log4rs, and I use rustyline for interation.