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
Verified it never flakes anymore:
go test ./internal/workerutil -run=TestWorkerCancelJobs -count=1000000 ok github.com/sourcegraph/sourcegraph/internal/workerutil 39.729s
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