Fix flaky cancel worker jobs test
Created by: eseliger
Sometimes the .After wasn't called when we advanced the clock, leading to it waiting from that point in time. BlockingAdvance waits for an .After handler to attach.
Closes https://github.com/sourcegraph/sourcegraph/issues/39749
Test plan
Verified it never flakes anymore:
go test ./internal/workerutil -run=TestWorkerCancelJobs -count=1000000
ok github.com/sourcegraph/sourcegraph/internal/workerutil 39.729s