reconsider posting CI comments on every test failure

Currently Kubernetes PRs get a _lot_ of comments from @k8s-ci-robot, many of which are to notify of failing tests. This tends to drown out actual human interactions.

I know this came up at some point in the past, but I'm having difficulty finding why we do this and I'd like to revisit it.

Right now prow deletes it's job-results comment every time results change, and posts a new comment if there are any failures. This can be very spammy (see below), and punishes reviewers and approvers for being subscribed to PRs with tons of excess notifications.

Most (all?) other CI on GitHub merely post to the "status" or "checks" APIs (https://help.github.com/en/articles/about-status-checks) which Prow / @k8s-ci-robot also does. We should consider only doing this instead of posting comments on every failure to reduce pressure on developers's notifications.

Additionally, the comments can be misleading with stale state in them https://github.com/kubernetes/test-infra/issues/4602

If we agree that this is reasonable, it should be relatively straightforward to make it possible in test-infra.