Fix flake unit test TestAddWithParser

[danilo-gemoli]

This PR is an attempt to fix #31313.
As far as I was able to understand the test pulled out the value from the generator (here) before the agent had the time to update it (here).
This solution might not be ideal still, due to the 10s time.Sleep but we need to go a pretty extensive refactor to make the test 100% deterministic.
Anyway, as far as I know the sleep has never been a problem so far, therefore we might be good this way.

[k8s-ci-robot]

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[k8s-ci-robot]

PR needs rebase.

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[jihoon-seo]

Note: The PR kubernetes-sigs/prow#110 supersedes this one,
so after kubernetes-sigs/prow#110 gets merged, this PR becomes okay to be closed.