keps/prod-readiness: add sig-testing/2539
keps/prod-readiness: add sig-testing/2539

This went to beta in v1.21 before PRR was mandated as required. It's a
tooling/process KEP that doesn't affect the contents of a kubernetes
release, so I expect this to be a rubber stamp

kubernetes/enhancements,kubernetes/enhancements,kubernetes/enhancements