Print test failures on INFO signal

[tenderlove]

Hi,

One thing I miss from minitest is if I have a slow test suite, and there is a failure, I can hit Ctrl-T to send a SIGINFO and it will print out the current failures with a stack trace. Then I can decide if I want to kill the process and fix those or just let it go.

I made a video to demonstrate.

I will try to make a PR for this, but I wanted to file a ticket a) so I don't forget, and b) in case someone can get to it before me.

Thanks!