Ugly, but effective

I can't think of a more effective test than this:

+ Run `rake test`        # => see pretty colors
+ Run `rake test | less` # => no colors in less
+ Run `rake test > out`  # => no garbage \e[32m output in the file

If you're going to update soon for minitest 5.0.0, this may be moot but it gives you an idea of what I'd like to do.