Basically, the asyncTest never returned. This manifested itself in the html view of the tests like this (left is success, right is where the test never returns–note the white):
Turns out that sinon-qunit adapter has sinon fake the browser timer, so setTimeout doesn’t work as expected.
To fix, just turn off the timer faking for a single test:
sinon.config.useFakeTimers = false to disable this for all tests.