[TIP] Tool shown at BoF for using either real or fake browsers?

[Ned Batchelder]

At the TiP BoF (98.5% awesome, btw), someone showed a tool for writing 
unit tests against a browser abstraction, and it could either use fake 
browsers (fast) or real browsers (accurate) using the same test code.  
What was that?  Am I remembering it right?

--Ned.