I was sure I had come across it before, as well as the 140 Character Web-App Challenge that has inspired it, but not paid much attention to either. What made a difference this time, however, was that earlier that day I had written a tiny test function for a disposable proof of concept that didn't warrant a full unit testing framework. I opened up my file from earlier that day, and after a few tweaks, tweeted the result.
Since then (mostly for fun) I decided to see if I could replicate all of the assertions from PHPUnit with my tiny test function and nothing more. It turns out, you can, so I gave tweetest a name, created a GitHub repo and even a website with some documentation to show others what could be done.