Make `Chaos.cpu_spin` to consume thread

What does this MR do?

This changes .cpu_spin to actually consume thread for the desired duration, by looking at CPU thread time.

@smcgivern @andrewn This change makes us actually to consume real cpu of given duration, previously in multi-threaded workflow it could end-up with cpu_s to be of a significantly less than duration.

Related to: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8334

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