Cancels the run when it fires.

Cancellation here means "stop and answer", not "throw": the browser drops the rest of its queue and the run resolves with whatever it had, marked aborted: true. The tests that did finish are still on the result, which is the point — a cancelled run that discarded its own findings would be no more useful than one that never started.

An already-aborted signal short-circuits: no browser is launched at all.