Add `goleak` detection to workhorse tests
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Updating the workhorse tests to look for [`goleak`](https://github.com/uber-go/goleak)s will ideally help us detect Goroutine leaks during development (and not in production 🙏).
Related: https://gitlab.com/gitlab-com/gl-infra/capacity-planning-trackers/gitlab-com/-/issues/1796#note_1956967107
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