Added test for Net_SSH2::_format_log()

This test is required for proving #205. Fix depends on how we will solve #212.

Also this code is against dry, maybe we should discuss about moving the `Net_SSH1::_format_log` and `Net_SSH2::_format_log` to a own class like `Net_SSH_LogFormatter`? Then we can write just one test instead of two. @terrafrost what do you think, what's the best way to solve this?