See https://github.com/ooni/probe/issues/2290
While there, notice that in such a case the priority selector would hang because of the WaitGroup, so get rid of the WaitGroup and accept that the priority selector is going to hang around for the whole duration of the measurement in some cases. The cancellable `measurer.go`'s context will cause the priority selector to eventually exit when we return from `measurer.go`'s `Run` method.
