Feature request: option to debug tests

[eltoder]

While developing tests it is quite handy to put in breakpoints or extra prints. At the moment, when a test is run by green, breakpoints do not work because stdin is not connected to the terminal. I couldn't find any option to preserve stdin. This means I have to switch to running the test without green. While not the end of the world, it's inconvenient. It would be great if green had a command-line option to support test debugging. It could do the following:

1. Leave stdin connected. Optionally complain if stdin is not a tty.
2. Don't capture stdout.
3. Limit the number of processes to 1.

I think that will make developing tests with green more pleasant.