Add `each()`

This completes most functionalities needed.

Breaking change: in 0.7, process.cwd() set to each case's path automatically.

This feature has been removed to reduce confusion.
process.cwd(d.casePath) if you need to do that.

At the end of each test, the process.cwd() is restored to the original, so you don't have to do the clean up. 🌷