If you want to do it with the whole page and talk only to the local code, then yes, I'd recommend Sinon. I think that's a much simpler solution than . . . creating an all new NIH framework!
I'd also recommend refactoring to a more mock-friendly way to do that countdown if you don't want to cover up all the internal logic.
If the timeout interval is loaded remotely from some API (and it probably is if you have reasonably configurable nag popups), then you can always mock that API call.
I'd also recommend refactoring to a more mock-friendly way to do that countdown if you don't want to cover up all the internal logic.
If the timeout interval is loaded remotely from some API (and it probably is if you have reasonably configurable nag popups), then you can always mock that API call.