Consider adding with-simulated-input package to testing section

Hi, I have a package that you might want to include in the handbook. It's called [with-simulated-input](https://github.com/DarwinAwardWinner/with-simulated-input), and its purpose is to enable automated non-interactive testing of interactive commands. I use it extensively in the [test suite for my other package, ido-completing-read+](https://github.com/DarwinAwardWinner/ido-completing-read-plus/blob/master/tests/test-ido-completing-read%2B.el), which is heavily focused on user interaction.